Some folks are trying to create a DFDL model for IMF (internet message format ... basically email) There is this header structure which really wants unordered sequences. Since we don't have those yet I told them to use an array of choice like this: <xs:element name="hdrArray" minOccurs="1" maxOccurs="25"> <xs:complexType> <xs:choice> <xs:element ref="TO_HDR"/> <xs:element ref="FROM_HDR" /> </xs:choice> ... But then there are these contraints, like there should be only one FROM address, but there can be several TO addresses. So, one might want to write a DFDL expression using an XPath-like query path in it. E.g., <xs:assert>{ dfdl:occursCount( hdrArray/FROM_HDR ) eq 1 }</xs:assert> Here's the question: Is the DFDL expression in this assert legal? I advised them that no, it is not, because hdrArray is an array and isn't being indexed, while FROM_HDR is not an array, but is being used as the argument to dfdl:occursCount. I think DFDL's expression language is simply not powerful enough to express these constraints. I'm just looking for affirmation or discussion if there is more to say here.