The <BELOW> element is used to draw a line, arrow, or curly bracket below the expression enclosed by this element. For example:
<below>X + Y</below> giving X + Y _____ <below sym=rarr>X + Y</below> giving X + Y ---->
You can also place an expression centered below the line or arrow with the SUP element or its shortref form, for example:
<above sym=cub>n(n - 1)(n - 2)&dots;(n - m + 1)</above> <sup><text>total of m factors</text></sup>
which would be rendered as (within limits of ascii art):
n(n - 1)(n - 2) ... (n - m + 1) \---------------v-------------/ total of m factors
I can't find the ISO entity names for under/over curly brackets!
Note: Don't include the & prefix, so <below sym="→"> is wrong!