Up: Table of Contents  Working Draft 6Jan98 
Besides tokens there are several families of MathML presentation elements. One family of elements deals with various "scripting" notations, such as subscript and superscript. Another family is concerned with matrices and tables. The remainder of the elements, discussed in this section, describe other basic notations such as fractions and radicals, or deal with general functions such as setting style properties and error handling.
An <mrow> element is used to group together any number of subexpressions, usually consisting of one or more <mo> elements acting as "operators" on one or more other expressions which are their "operands".
Several elements automatically treat their arguments as if they were contained in an <mrow> element. See the discussion of the <mfenced> element, and of inferred <mrow>s in Section 3.1.3.
None (except the attributes allowed for all MathML elements, listed in section 2.3.4).
<mrow> elements are typically rendered visually as a horizontal row of their arguments, left to right in the order in which the arguments occur, or audibly as a sequence of renderings of the arguments. The description in Section 3.2.4 of suggested rendering rules for <mo> elements assumes that all horizontal spacing between operators and their operands is added by the rendering of <mo> elements (or, more generally, embellished operators), not by the rendering of the <mrow>s they are contained in.
MathML is designed to allow renderers to automatically linebreak expressions (that is, to break excessively long expressions into several lines), without requiring authors to specify explicitly how this should be done. This is because linebreaking positions can't be chosen well without knowing the width of the display device and the current font size, which for many uses of MathML will not be known except by the renderer at the time of each rendering.
Determining good positions for linebreaks is complex, and rules for this are not described here; whether and how it is done is up to each MathML renderer. Typically, linebreaking will involve selection of "good" points for insertion of linebreaks between successive arguments of <mrow> elements.
Although MathML does not require linebreaking or specify a particular linebreaking algorithm, it has several features designed to allow such algorithms to produce good results. These include the use of special entities for certain operators, including invisible operators (see Section 3.2.4), or for providing hints related to linebreaking when necessary (see Section 3.2.5), and the ability to use nested <mrow>s to describe subexpression structure (see below).
MathML renderers are required to treat an <mrow>
element containing exactly one argument as equivalent in all ways
to the single argument occurring alone, provided there are no
attributes on the <mrow> element's begin tag. If there
are attributes on the <mrow> element's begin tag, no
requirement of equivalence is imposed. This equivalence condition
is intended to simplify the implementation of MathML software such
as templatebased authoring tools. It directly affect the
definition of
embellished operator and spacelike element and the rules
for determining the default value of the form attribute of
an <mo> element; see Sections 3.2.4 and 3.2.6. See
also the discussion of equivalence of MathML expressions in Chapter 7.
Subexpressions should be grouped by the document author in the
same way as they are grouped in the mathematical interpretation of
the expression; that is, according to the underlying "syntax tree"
of the expression. Specifically, operators and their mathematical
arguments should occur in a single <mrow>; more than
one operator should occur directly in one <mrow> only
when they can be considered to act together on the interleaved
arguments, e.g. for chains of relational operators, or for
sequences of terms separated by +
and 
.
Operators which act together on interleaved arguments are listed in
the same group in the operator dictionary provided in Appendix C. In all other cases, nested
<mrow>s should be used.
Proper grouping has several purposes: it improves display by possibly affecting spacing; it allows for more intelligent linebreaking and indentation; and it simplifies possible semantic interpretation of presentation elements by computer algebra systems, and audio renderers.
Although improper grouping will sometimes result in suboptimal renderings, and will often make interpretation other than pure visual rendering difficult or impossible, any grouping of expressions using <mrow> is allowed in MathML syntax; that is, renderers should not assume the rules for proper grouping will be followed.
<mrow>
The proper encoding of (x,
y) furnishes a less obvious example of nesting
<mrow>s:
<mrow>
<mn> 2
</mn>
<mo>
</mo>
<mi> x
</mi>
</mrow>
<mo> + </mo>
<mi> y </mi>
<mo>  </mo>
<mi> z </mi>
</mrow>
<mrow>
<mo> ( </mo>
<mrow>
<mi> x
</mi>
<mo> ,
</mo>
<mi> y
</mi>
</mrow>
<mo> ) </mo>
</mrow>
In this case, a nested <mrow> is required inside
the parentheses, since parentheses and commas, thought of as fence
and separator "operators" do not act together on interleaved
arguments.
The <mfrac> element is used for fractions. It can also be used to mark up fractionlike objects such as binomial coefficients and Legendre symbols. The syntax for <mfrac> is
<mfrac> numerator denominator </mfrac>
Name  values  default  
linethickness  number  1 (rule thickness) 
The linethickness attribute indicates the thickness of the horizontal "fraction bar", or "rule", typically used to render fractions. A fraction with linethickness="0" renders without the bar, and might be used within binomial coefficients. A linethickness greater than one might be used with nested fractions. These cases are shown below:
The <mfrac> element sets displaystyle to "false", or if it was already false increments scriptlevel by 1, within numerator and denominator. These attributes are inherited by every element from its rendering environment, but can be set explicitly only on the <mstyle> element. (See Section 3.3.4.)
The examples shown above can be represented in MathML as:
<mrow>
<mo> ( </mo>
<mfrac linethickness="0">
<mi> a
</mi>
<mi> b
</mi>
</mfrac>
<mo> ) </mo>
</mrow>
<mfrac linethickness="2">
<mfrac>
<mi> a
</mi>
<mi> b
</mi>
</mfrac>
<mfrac>
<mi> c
</mi>
<mi> d
</mi>
</mfrac>
</mfrac>
A more generic example is:
<mfrac>
<mrow>
<mn> 1
</mn>
<mo> +
</mo>
<msqrt>
<mn>
5 </mn>
</msqrt>
</mrow>
<mn> 2 </mn>
</mfrac>
These elements draw radicals. The <msqrt> element is used for square roots, while the <mroot> element is used to draw radicals with indices, e.g. a cube root. The syntax for these elements is
<msqrt> base </msqrt><mroot> base index </mroot>
The <mroot> element requires exactly 2 arguments. However, <msqrt> accepts any number of arguments; if this number is not 1, its contents are treated as a single "inferred <mrow>" containing its arguments, as described in Section 3.1.3.
The <mroot> element increments scriptlevel
by 2, and sets displaystyle to "false", within index,
but leaves both attributes unchanged within base. The
<msqrt> element leaves both attributes unchanged within
all its arguments. These attributes are inherited by every element
from its rendering environment, but can be set explicitly only on
<mstyle>. (See Section
3.3.4.)
The <mstyle> element is used to make style changes which affect the rendering of its contents. <mstyle> can be given any attribute accepted by any MathML presentation element; it can also be given certain special attributes listed below.
The <mstyle> element accepts any number of arguments. If this number is not 1, its contents are treated as a single "inferred <mrow>" formed from all its arguments, as described in Section 3.1.3.
Loosely speaking, the effect of the <mstyle> element is to set the default value of an attribute for its children elements. Style changes work in one of two ways, depending on whether the attribute being set inherits a value from the environment, or has a default value. In both cases, the net effect is the same, but implementors should be aware that the machanism is slightly different. The two cases are:
Note that attribute values inherited from an <mstyle>, in either manner, affect a given element in the <mstyle>'s content only if that attribute is not given a value in that element's begin tag. In this case, the attribute specified on the tag overrides the inherited value. The only exception to this rule is when the value given on the tag is documented as specifying an incremental change to the value inherited from that element's context or rendering environment.
There is one exceptional element, <mpadded>, whose attributes cannot be set with <mstyle>. When the "dimensional" attributes width, height and depth are specified with an <mstyle>, they apply only to the <mspace/> element. Similarly, when lspace is set with <mstyle>, it only applies to the <mo;> element.
As stated above, <mstyle> accepts all attributes of all MathML presentation elements. Additionally, <mstyle> can be given the following special attributes which are implicitly inherited by every MathML element as part of its rendering environment:
Name  values  default  
scriptlevel  ['+'  ''] unsignedinteger  inherited  
displaystyle  true  false  inherited  
scriptsizemultiplier  number  0.71  
scriptminsize  number (points)  8  
fontcolor  #rrggbb  inherited  
background  #rrggbb  inherited 
MathML uses two attributes, displaystyle and scriptlevel, to control orthogonal presentation features that TeX encodes into one "style" attribute with values \displaystyle, \textstyle, \scriptstyle, and \scriptscriptstyle. The corresponding values of displaystyle and scriptlevel for those TeX styles would be "true" and "0", "false" and "0", "false" and "1", and "false" and "2", respectively.
The main effect of the displaystyle attribute is that it determines the effect of other attributes such as the largeop and movablescripts attributes of <mo>. The main effect of the scriptlevel attribute is to control the font size. Typically, the higher the scriptlevel, the smaller the font size. (Nonvisual renderers can respond to the font size in an analogous way for their medium.) More sophisticated renderers may also choose to use these attributes in other ways, such as rendering expressions with displaystyle="false" in a more vertically compressed manner.
These attributes are given initial values for the outermost expression of an instance of MathML based on its rendering environment. A short list of exceptional layout schemata described below modify these values for some of their subexpressions. Otherwise, values are determined by inheritance whenever they are not directly specified on a given element's start tag.
For an instance of MathML embedded in a textual data format (such as HTML) in "display" mode, i.e. in place of a paragraph, displaystyle = "true" and scriptlevel = "0" for the outermost expression of the embedded MathML; if the MathML is embedded in "inline" mode, i.e. in place of a character, displaystyle = "false" and scriptlevel = "0" for the outermost expression. See Chapter 7 for further discussion of the distinction between "display" and "inline" embedding of MathML and how this can be specified in particular instances. In general, a MathML renderer may determine these initial values in whatever manner is appropriate for the location and context of the specific instance of MathML it is rendering, or if it has no way to determine this, based on the way it is most likely to be used; as a last resort it is suggested that it use the most generic values displaystyle = "true" and scriptlevel = "0".
The MathML layout schemata which typically display some of their arguments in smaller type or with less vertical spacing, namely the elements for scripts, fractions, radicals, and tables or matrices, set displaystyle to "false", and in some cases increase scriptlevel, for those arguments. The new values are inherited by all subexpressions within those arguments, unless they are overriden.
The specific rules by which each element modifies displaystyle and/or scriptlevel are given in the specification for each element which does so; the complete list of elements which modify either attribute are: the "scripting" elements <msub>, <msup>, <msubsup>, <munder>, <mover>, <munderover>, and <mmultiscripts>; and the elements <mfrac>, <mroot>, and <mtable>.
When <mstyle> is given a scriptlevel attribute with no sign, it sets the value of scriptlevel within its contents to the value given, which must be a nonnegative integer. When the attribute value consists of a sign followed by an integer, the value of scriptlevel is incremented (for '+') or decremented (for '') by the amount given. The incremental syntax for this attribute is an exception to the general rules for setting inherited attributes using <mstyle>, and is not allowed by any other attribute on <mstyle>.
Whenever the scriptlevel is changed, either automatically or by being explicitly incremented, decremented, or set, the current font size is multiplied by the value of scriptsizemultiplier to the power of the change in scriptlevel. For example, if scriptlevel is increased by 2, the font size is multiplied by scriptsizemultiplier twice in succession; if scriptlevel is explicitly set to 2 when it had been 3, the font size is divided by scriptsizemultiplier.
The default value of scriptsizemultiplier is less than one (in fact, it is approximately the square root of 1/2), resulting in a smaller font size with increasing scriptlevel. To prevent scripts from becoming unreadably small, the font size is never allowed to go below the value of scriptminsize as a result of a change to scriptlevel, though it can be set to a lower value using the fontsize attribute (Section 3.2.1) on <mstyle> or on token elements. If a change to scriptlevel would cause the font size to become lower than scriptminsize using the above formula, the font size is instead set equal to scriptminsize within the subexpression for which scriptlevel was changed.
Explicit changes to the fontsize attribute have no effect on the value of scriptlevel.
If scriptlevel is changed incrementally by an <mstyle> element which also sets certain other attributes, the overall effect of the changes may depend on the order in which they are processed. In such cases, the attributes in the following list should be processed in the following order, regardless of the order in which they occur in the XMLformat attribute list of the <mstyle> begin tag: scriptsizemultiplier, scriptminsize, scriptlevel, fontsize.
Note that scriptlevel can, in principle, attain any integral value by being decremented sufficiently, even though it can only be explicitly set to nonnegative values. Negative values of scriptlevel generated in this way are legal and should work as described, generating font sizes larger than those of the surrounding expression. Since scriptlevel is initially 0 and never decreases automatically, it will always be nonnegative unless it is decremented past 0 using <mstyle>.
Explicit decrements of scriptlevel after the font size has been limited by scriptminsize as described above would produce undesirable results. This might occur, for example, in a representation of a continued fraction, in which the scriptlevel was decremented for part of the denominator back to its value for the fraction as a whole, if the continued fraction itself was located in a place which had a high scriptlevel. To prevent this problem, MathML renderers should, when decrementing scriptlevel, use as the initial font size the value the font size would have had if it had never been limited by scriptminsize. They should not, however, ignore the effects of explicit settings of fontsize, even to values below scriptminsize.
Since MathML renderers may be unable to make use of arbitrary font sizes with good results, may wish to modify the mapping from scriptlevel to fontsize to produce better renderings in their judgement. In particular, if fontsizes have to be rounded to available values, or limited to values within a range, the details of how this is done are up to the renderer. Renderers should, however, ensure that a series of incremental changes to scriptlevel resulting in its return to the same value for some subexpression that it had in a surrounding expression results in the same fontsize for that subexpression as for the surrounding expression.
The fontcolor attribute controls the color in which the content of tokens is rendered. Additionally, when inherited from <mstyle> or from a MathML expression's rendering environment, it controls the color of all other drawing by MathML elements, including the lines or radical signs that can be drawn by <mfrac>, <mtable>, or <msqrt>.
Note that the background attribute, also implicitly inherited by every MathML subexpression, controls the background color used in rendering that subexpression, even though it is not explicitly specifiable on any element except <mstyle>. The suggested MathML visual rendering rules do not define the precise extent of the region whose background color is affected by using this attribute on <mstyle>, except that, when <mstyle>'s content does not have negative dimensions and its drawing region is not overlapped by other drawing due to surrounding negative spacing, this region should lie behind all the drawing done to render the content of the <mstyle>, but not lie behind any of the drawing done to render surrounding expressions. The effect of overlap of drawing regions caused by negative spacing on the extent of the region colored by the background attribute is not defined by these rules.
The values of fontcolor and background are specified as a string consisting of '#' followed without intervening whitespace by 2digit hexadecimal values for the red, green, and blue components, respectively, of the desired color, with each component value ranging from 00 (not present) to FF (fully present), as in the color specifications of the BGCOLOR attribute in HTML. The hexadecimal digits are not casesensitive.
The example of limiting the stretchiness of a parenthesis shown in the section on <mo>,
<mrow>
<mo maxsize="1"> (
</mo>
<mfrac> <mi> a
</mi> <mi> b </mi> </mfrac>
<mo maxsize="1"> )
</mo>
</mrow>
can be rewritten using <mstyle> as:
<mstyle maxsize="1">
<mrow>
<mo> (
</mo>
<mfrac>
<mi> a </mi> <mi> b </mi>
</mfrac>
<mo> )
</mo>
</mrow>
</mstyle>
The <merror> element displays its contents as an "error message". This might be done, for example, by displaying the contents in red, flashing the contents, or changing the background color. The contents can be any expression or expression sequence.
The <merror> accepts any number of arguments; if this number is not 1, its contents are treated as a single "inferred <mrow>" as described in Section 3.1.3.
The intent of this element is to provide a standard way for programs that generate MathML from other input to report syntax errors in their input. Since it is anticipated that preprocessors that parse input syntaxes designed for easy hand entry will be developed to generate MathML, it is important that they have the ability to indicate that a syntax error occurred at a certain point. See Section 7.2.2.
The suggested use of <merror> for reporting syntax errors is for a preprocessor to replace the erroneous part of its input with an <merror> element containing a description of the error, while processing the surrounding expressions normally as far as possible. By this means, the error message will be rendered where the erroneous input would have appeared, had it been correct; this makes it easier for an author to determine from the rendered output what portion of the input was in error.
No specific error message format is suggested here, but as with error messages from any program, the format should be designed to make as clear as possible (to a human viewer of the rendered error message) what was wrong with the input and how it can be fixed. If the erroneous input contains correctly formatted subsections, it may be useful for these to be preprocessed normally and included in the error message (within the contents of the <merror> element), taking advantage of the ability of <merror> to contain arbitrary MathML expressions rather than only text.
None (except the attributes allowed for all MathML elements, listed in section 2.3.4).
If a MathML syntaxchecking preprocessor received the input
<mfraction>
<mrow> <mn> 1 </mn>
<mo> + </mo> <msqrt> <mn> 5 </mn>
</msqrt> </mrow>
<mn> 2 </mn>
</mfraction>
which contains the nonMathML element <mfraction> (presumably in place of the MathML element <mfrac>), it might generate the error message
<merror>
<mtext> Unrecognized element:
<mfraction>; arguments were: </mtext>
<mrow> <mn> 1 </mn>
<mo> + </mo> <msqrt> <mn> 5 </mn>
</msqrt> </mrow>
<mtext> and
</mtext>
<mn> 2 </mn>
</merror>
Note that the preprocessor's input is not, in this case, valid
MathML, but the error message it outputs is valid MathML.
An <mpadded> element renders the same as its content, but with its overall size and other dimensions (such as baseline position) modified according to its attributes. The <mpadded> element does not rescale (stretch or shrink) its content; its only effect is to modify the apparent size and position of the "bounding box" around its content, so as to affect the relative positioning of surrounding elements. The name reflects the use of <mpadded> to effectively add "padding", or extra space, around its content. If the "padding" is negative, it is possible for the content of <mpadded> to be rendered outside the <mpadded> element's bounding box; see below for warnings about several potential pitfalls of this effect.
The <mpadded> element accepts any number of arguments; if this number is not 1, its contents are treated as a single "inferred <mrow>" as described in Section 3.1.3.
It is suggested that audio renderers add (or shorten) time delays based on the attributes representing horizontal space ( width and lspace).
Name  values  default  
width  [ +   ] unsignednumber [ % ] [ width  lspace  height  depth ]  same as content  
lspace  [ +   ] unsignednumber [ % ] [ width  lspace  height  depth ]  0  
height  [ +   ] unsignednumber [ % ] [ width  lspace  height  depth ]  same as content  
depth  [ +   ] unsignednumber [ % ] [ width  lspace  height  depth ]  same as content 
These attributes modify the dimensions of the "bounding box" of the <mpadded> element. Depending on the format of the attribute, a value may be set, incremented or decremented. Values may be specified either as multiples or as percentages of one of the dimensions of the normal rendering of the element's content, or in ems or exs.
If an attribute value begins with a +
or

sign, its value specifies an increment or decrement.
Otherwise, the attribute is set directly to the specified value.
Note that the +
and 
do not mean that
the following numeric value is positive or negative. In particular,
these attributes cannot be directly set to a negative value.
Numeric values for these attributes can be specified in several
formats. Each format begins with an unsignednumber, which
may be followed by an optional %
sign, and/or an
optional "pseudounit". The valid "pseudounits" are
width
, lspace
, height
, or
depth
.
If a pseudounit is given, it represents the value of the
samenamed dimension of the <mpadded> element's
content (not of the <mpadded> element itself), and the
unsignednumber multiplies this value. If %
is
present, the unsignednumber specifies a percentage of the
value of the pseudounit. Thus depth="100% height"
and
depth="1.0 height"
both set the depth of the
<mpadded> element to the height of its content.
If no pseudounit is given, but a %
sign follows
the unsignednumber, the pseudounit with the same name as
the attribute is assumed. Thus depth="105%"
sets the
depth to 1.05 times its former value, and either
depth="+100%"
or depth="200%"
doubles the
depth.
If neither %
nor a pseudounit is given, the
numeric value is taken as being in ems (the width of an 'M' in the
current font and fontsize) for horizontal dimensions, or in exs
(the height of an 'x' in the current font and fontsize) for
vertical dimensions. Thus depth="+1"
increases the
depth by 1 ex.
Dimensions that would be positive if rendered normally cannot be made negative using <mpadded>; a positive dimension is set to 0 if it would otherwise become negative. Dimensions which are initially 0 can be made negative, but this should generally be avoided. See the warnings below on the use of negative spacing for "tweaking" or conveying meaning.)
The rules given above imply that all of the following attribute settings have the same effect, which is to leave the content's dimensions unchanged:
<mpadded width="+0"> ...
</mpadded>
<mpadded width="+0%"> ... </mpadded>
<mpadded width="0"> ... </mpadded>
<mpadded width=" 0 height"> ...
</mpadded>
<mpadded width="100%"> ... </mpadded>
<mpadded width="100% width"> ...
</mpadded>
<mpadded width="1 width"> ...
</mpadded>
<mpadded width="1.0 width"> ...
</mpadded>
<mpadded> ... </mpadded>
See the Glossary given in Appendix B for further information about some of the typesetting terms used here.
The width attribute refers to the overall horizontal width of a bounding box. By default, the content of a <mpadded> should be rendered flush with the left edge of the bounding box. Thus, increasing the width alone effectively adds space on the right edge of the box.
The lspace attribute refers to the amount of space between the left edge of a bounding box and where the rendering of its contents actually begins. Unlike the other dimensions, lspace does not correspond to a real property of a bounding box, but exists only transiently during the computations done by each instance of <mpadded>. It is provided so that there is a way to add space on the left edge of a bounding box.
The rationale behind using width and lspace to control horizontal padding instead of a more symmetric attributes, such as a hypothetical rspace and lspace, is that in practice, it is important to have a "width" pseudo unit.
The height attribute refers to the amount of vertical space between the baseline (the line along the bottom of most letter glyphs in normal text rendering) and the top of the bounding box.
The depth attribute refers to the amount of vertical space between the bottom of the bounding box and the baseline.
MathML renderers should ensure that, except for the effects of
the attributes, relative spacing between the contents of
<mpadded> and surrounding MathML elements is not modified
by replacing an <mpadded> element with an
<mrow> element with the same content. This holds even if
linebreaking occurs within the <mpadded> element.
However, if an <mpadded> element with nondefault
attribute values is subjected to linebreaking, MathML does not
define how its attributes or rendering interact with the
linebreaking algorithm.
A likely temptation for the use of the <mpadded> and <mspace/> elements (and perhaps also <mphantom> and <mtext>) will be for an author to improve the spacing generated by a specific renderer by slightly modifying it in specific expressions  i.e., to "tweak" the rendering.
Authors are strongly warned that different MathML renderers may use different spacing rules for computing the relative positions of rendered symbols in expressions which have no explicit modifications to their spacing; if renderer B improves upon renderer A's spacing rules, explicit spacing added to improve the output quality of renderer A may produce very poor results in renderer B, very likely worse than without any "tweaking" at all.
Even when a specific choice of renderer can be assumed, its spacing rules may be improved in successive versions, so that the effect of tweaking in a given MathML document may grow worse with time. Also, when style sheet mechanisms are extended to MathML, even one version of a renderer may use different spacing rules for users with different style sheets.
Therefore, it is suggested that MathML markup never use <mpadded> or <mspace/> elements to tweak the rendering of specific expressions, unless the MathML is generated solely to be viewed using one specific version of one MathML renderer, using one specific style sheet (if style sheets are available in that renderer).
In cases where the temptation to improve spacing proves too
strong, careful use of <mpadded>,
<mphantom>, or the alignment elements (Section 3.5.4) may give more portable
results than the direct insertion of extra space using
<mspace/> or <mtext>. Advice given to the
implementors of MathML renderers might be still more productive, in
the long run.
MathML elements which permit "negative spacing", namely <mspace/>, <mpadded>, and <mtext>, could in theory be used to simulate new notations or "overstruck" characters by the visual overlap of the renderings of more than one MathML subexpression.
This practice is strongly discouraged in all situations, for the following reasons:
More generally, any construct which uses spacing to convey mathematical meaning, rather than simply as an aid to viewing expression structure, is discouraged. That is, the constructs which are discouraged are those which would be interpreted differently by a human viewer of rendered MathML if all explicit spacing was removed.
If such constructs are used in spite of this warning, they should be enclosed in a <semantics> element which also provides an additional MathML expression which can be interpreted in a standard way. ##[editors: I changed the link from the form with underscores in my earlier email, HREF="chapter4.html#define_semantics_element", to the form with dashes, HREF="chapter4.html#definesemanticselement"; this should be made to exist in Chapter 4, and if it is already there, changed from underscores to dashes.]
For example, the MathML expression
<mrow>
<mpadded width="0"> <mi>
C </mi> </mpadded>
<mspace width="0.3"/>
<mtext>  </mtext>
</mrow>
forms an overstruck symbol in violation of the policy stated above; it might be intended to represent the set of complex numbers for a MathML renderer which lacks support for the standard symbol used for this purpose. This kind of construct should always be avoided in MathML, for the reasons stated above; indeed, it should never be necessary for standard symbols, since a MathML renderer with no better method of rendering them is free to use overstriking internally, so that it can still support general MathML input.
However, if for whatever reason such a construct is used in MathML, it should always be enclosed in a <semantics> element such as
<semantics>
<mrow>
<mpadded
width="0"> <mi> C </mi> </mpadded>
<mspace
width="0.3"/>
<mtext> 
</mtext>
</mrow>
<xmlannotation
encoding="mathml">
<mi>
&DoubleStruckCapitalC; </mi>
</xmlannotation>
</semantics>
which provides an alternative, standard encoding for the desired
symbol, which is much more easily interpreted than the construct
using negative spacing. (The alternative encoding in this example
uses MathML presentation elements; the content elements described
in Chapter 4 should also be
considered.)
The <mphantom> element renders invisibly, but with the same size and other dimensions, including baseline position, that its contents would have if they were rendered normally. <mphantom> can be used to align parts of an expression by invisibly duplicating subexpressions.
The <mphantom> element accepts any number of arguments; if this number is not 1, its contents are treated as a single "inferred <mrow>" formed from all its arguments, as described in Section 3.1.3.
It is suggested that audio renderers render <mphantom> elements in an analogous way for their medium, by rendering them as silence of the same duration as the normal rendering of their contents.
None (except the attributes allowed for all MathML elements, listed in section 2.3.4).
Note that it is possible to wrap both an <mphantom>
and an <mpadded> element around one MathML expression,
as in <mphantom><mpadded
attributesettings> ...
</mpadded></mphantom>
, to change its size and
make it invisible at the same time.
MathML renderers should ensure that the relative spacing between the contents of an <mphantom> element and the surrounding MathML elements is the same as it would be if the <mphantom> element were replaced by an <mrow> element with the same content. This holds even if linebreaking occurs within the <mphantom> element.
For the above reason, <mphantom> is not considered spacelike (Section 3.2.6) unless its content is spacelike, since the suggested rendering rules for operators are affected by whether nearby elements are spacelike. Even so, the warning about the legal grouping of spacelike elements may apply to uses of <mphantom>.
There is one situation where the preceding rule for rendering an <mphantom> may not give the desired effect. When an <mphantom> is wrapped around a subsequence of the arguments of an <mrow>, the default determination of the form attribute for an <mo> element within the subsequence can change. (See the default value of the form attribute described in Section 3.2.4.) It may be necessary to add an explicit form attribute to such an <mo> in these cases. This is illustrated in the following example.
In this example, <mphantom> is used to ensure alignment of corresponding parts of the numerator and denominator of a fraction:
<mfrac>
<mrow>
<mi> x
</mi>
<mo> +
</mo>
<mi> y
</mi>
<mo> +
</mo>
<mi> z
</mi>
</mrow>
<mrow>
<mi> x
</mi>
<mphantom>
<mo
form="infix"> + </mo>
<mi>
y </mi>
</mphantom>
<mo> +
</mo>
<mi> z
</mi>
</mrow>
</mfrac>
This would render as something like
x + y + z

x + z
rather than as
x + y + z

x + z
The explicit attribute setting form="infix" on the
<mo> element inside the <mphantom> sets the
form attribute to what it would have been in the absence of
the surrounding <mphantom>. This is necessary since
otherwise, the +
sign would be interpreted as a prefix
operator, which might have slightly different spacing.
Alternatively, this problem could be avoided without any
explicit attribute settings, by wrapping each of the arguments
<mo>+</mo>
and
<mi>y</mi>
in its own <mphantom>
element, i.e.
<mfrac>
<mrow>
<mi> x
</mi>
<mo> +
</mo>
<mi> y
</mi>
<mo> +
</mo>
<mi> z
</mi>
</mrow>
<mrow>
<mi> x
</mi>
<mphantom>
<mo>
+ </mo>
</mphantom>
<mphantom>
<mi>
y </mi>
</mphantom>
<mo> +
</mo>
<mi> z
</mi>
</mrow>
</mfrac>
The <mfenced> element provides a convenient form in which to express common constructs involving fences (i.e. braces, brackets, and parentheses), possibly including separators (such as comma) between the arguments.
For example, <mfenced> <mi>x</mi>
</mfenced>
renders as "(x)" and is equivalent
to
<mrow> <mo> ( </mo>
<mi>x</mi> <mo> ) </mo>
</mrow>
,
and <mfenced> <mi>x</mi>
<mi>y</mi> </mfenced>
renders as "(
x, y)" and is equivalent to
<mrow>
<mo> ( </mo>
<mrow> <mi>x</mi>
<mo>,</mo> <mi>y</mi> </mrow>
<mo> ) </mo>
</mrow>
Individual fences or separators are represented using <mo> elements, as described in Section 3.2.4. Thus, any <mfenced> element is completely equivalent to an expanded form described below; either form can be used in MathML, at the convenience of an author or of a MathMLgenerating program. A program which processes MathML is not permitted to behave differently when receiving the expanded or unexpanded form.
In general, an <mfenced> element can contain zero or more arguments, and will enclose them between fences in an <mrow>; if there is more than one argument, it will insert separators between adjacent arguments, using an additional nested <mrow> around the arguments and separators for proper grouping (Section 3.3.1). The general expanded form is shown below. The fences and separators will be parentheses and comma by default, but can be changed using attributes, as shown in the following table.
Name  values  default  
open  string  (  
close  string  )  
separators  character *  , 
A generic <mfenced> element, with all attributes explicit, looks as follows:
<mfenced open="openingfence"
close="
closingfence"
separators="
sep#1 sep#2 ... sep#(n1)" >
arg#1
...
arg#n
</mfenced>
The openingfence and closingfence are arbitrary strings. (Whitespace in them is trimmed and collapsed as described in section 2.3.3, and is not generally useful anyway.)
The value of separators is a sequence of zero or more
separator characters (or entity references), optionally separated
by whitespace. Each sep#i
consists of exactly
one character or entity reference. Thus,
separators=",;"
is equivalent to separators=" , ;
"
.
The general <mfenced> element shown above is equivalent to the following expanded form:
<mrow>
<mo fence="true">
openingfence </mo>
<mrow>
arg#1
<mo
separator="true"> sep#1 </mo>
...
<mo
separator="true"> sep#(n1) </mo>
arg#n
</mrow>
<mo fence="true">
closingfence </mo>
</mrow>
Each argument except the last is followed by a separator. The inner <mrow> is added for proper grouping, as described in Section 3.3.1.
When there is only one argument, the above form has no
separators; since <mrow> arg#1
</mrow>
is equivalent to arg#1
(as
described in Section 3.3.1),
this case is also equivalent to:
<mrow>
<mo fence="true">
openingfence </mo>
arg#1
<mo fence="true">
closingfence </mo>
</mrow>
If there are too many separator characters, the extra ones are
ignored. If separators characters are given, but there are too few,
the last one is repeated as necessary. Thus, the default value of
separators="," is equivalent to separators=",,",
separators=",,,", etc. If there are no separator characters
provided but some are needed, for example if separators=" "
or "" and there is more than one argument, then no separator
elements are inserted at all  that is, the elements <mo
separator="true"> sep#i </mo>
are left out
entirely. Note that this is different from inserting separators
consisting of <mo> elements with empty content.
Finally, for the case with no arguments, i.e.
<mfenced open="openingfence"
close="
closingfence"
separators="
anything" >
</mfenced>
the equivalent expanded form is defined to include just the fences within an <mrow>:
<mrow>
<mo fence="true">
openingfence </mo>
<mo fence="true">
closingfence </mo>
</mrow>
Note that not all "fenced expressions" can be encoded by an <mfenced> element. Such exceptional expressions include those with an "embellished" separator or fence or one enclosed in an <mstyle> element, a missing or extra separator or fence, or a separator with multiple content characters. In these cases, it is necessary to encode the expression using an appropriately modified version of an expanded form. As discussed above, it is always permissible to use the expanded form directly, even when it is not necessary. In particular, authors cannot be guaranteed that MathML preprocessors won't replace occurrences of <mfenced> with equivalent expanded forms.
Note that the equivalent expanded forms shown above include attributes on the <mo> elements which identify them as fences or separators. Since the most common choices of fences and separators already occur in the operator dictionary with those attributes, authors would not normally need to specify those attributes explicitly when using the expanded form directly. Also, the rules for the default form attribute (Section 3.2.4) cause the opening and closing fences to be effectively given the values form="prefix" and form="postfix" respectively, and the separators to be given the value form="infix".
Note that it would be incorrect to use <mfenced>
with a separator of, for instance, "+", as an abbreviation for an
expression using "+" as an ordinary operator, e.g.
<mrow> <mi>x</mi> <mo>+</mo>
<mi>y</mi> <mo>+</mo>
<mi>z</mi> </mrow>
. This is because the
+
signs would be treated as separators, not infix
operators. That is, it would render as if they were marked up as
<mo separator="true">+</mo>
, which might
therefore render inappropriately.
(a+b)
<mfenced>
<mrow>
<mi> a
</mi>
<mo> +
</mo>
<mi> b
</mi>
</mrow>
</mfenced>
Note that the above <mrow> is necessary so that the <mfenced> has just one argument. Without it, this would render incorrectly as "(a, +, b)".
[0,1)
<mfenced separators="[)">
<mn> 0 </mn>
<mn> 1 </mn>
</mfenced>
f(x,y)
<mrow>
<mi> f </mi>
<mo>
</mo>
<mfenced>
<mi> x
</mi>
<mi> y
</mi>
</mfenced>
</mrow>