SML does not define "user-defined constraints". 

Proposal:

1. Change section 5.0 from:

XML Schema supports a number of built-in grammar-based constraints but it does not support a language for defining arbitrary rules for constraining the structure and content of documents. Schematron [ISO/IEC 19757-3] is an ISO/IEC standard for defining assertions concerning a set of XML documents. SML uses a profile of the Schematron schema to add support for user-defined constraints. SML uses XPath, augmented with the smlfn:deref()  extension function, as its constraint language.

to:

XML Schema supports a number of built-in grammar-based constraints but it does not support a language for defining arbitrary rules for constraining the structure and content of documents. Schematron [ISO/IEC 19757-3] is an ISO/IEC standard for defining assertions concerning a set of XML documents. SML uses Schematron to add support for additional model constraints not supported in XML Schema. 

2. Section 5.1, 2nd paragraph, change:
User-defined constraints can be specified using...

to:
Constraints can be specified using...

3. Appendix B, bullet point 3, change:
This model definition document sample illustrates the use of the following SML extensions:

   1. SML references
   2. key and keyref constraints
   3. User-defined constraints

to:
This model definition document sample illustrates the use of the following SML extensions:

   1. SML references
   2. key and keyref constraints
   3. Schematron constraints