Union types

[Srdjan Djuricic]

Hello,

I have a question regarding union types and how to deal with them.  The
simpleType in question is a union of "time dateTime anyURI decimal date
gYearMonth gYear".  When a user performs a query they specify an element
name along with the literal value to do the comparison.  Based on the
schema we find out the element's primitive type and do a cast on the
input literal's value.  For example 

pbs:myProp = xs:dateTime("1999-12-31T12:00:00") 

The problem occurs if myProp is a simpleType with an union.  I do not
know how to cast the input data.  I tried using "instance of" on the
literal value operator but quickly found out that "1999-12-31T12:00:00"
instance of xs:dateTime yields false.  If I don't do any casting I get
an exception.  

Are there any suggestion as to how to deal with this issue?

Thanks,

Srdjan