2.2.9 [XML] Section 3.2.1, Element Content
Validity constraint: Proper Group/PE Nesting Parameter-entity replacement text MUST be properly nested with parenthesized groups. That is to say, if either of the opening or closing parentheses in a choice, seq, or Mixed construct is contained in the replacement text for a parameter entity, both MUST be contained in the same replacement text. For interoperability, if a parameter-entity reference appears in a choice, seq, or Mixed construct, its replacement text SHOULD contain at least one non-blank character, and neither the first nor last non-blank character of the replacement text SHOULD be a connector (| or ,).
MSXML3 and MSXML6
The following clarifications apply:
Parameter-entity replacement text can be blank if it is valid as defined in the DTD.
Parameter-entity replacement text can have the characters '|' or ',' at the beginning or end of the string if it is valid as defined in the DTD.