Use of fn:function in an expression with no xmlns:fn="..." in the file should be an SDE

When a DFDL expression is compiled, function names should be scrutinized, and the prefixes should have to be defined.

I believe right now whether you write fn:true() or just true() it will evaluate correctly. This is wrong. If there is a xmlns="..." then just true() should be the true function in that namespace, which probably doesn't exist.