It appears that it ain't as free as it should be:

   Person( age > $age * 2 )
Unable to return Declaration for identifier '$age * 2' : [Rule name='exa1']

   Person( isAdult() )
Unable to Analyse Expression isAdult():
[Error: no such identifier: isAdult]

   Book( "Jones" memberOf authors )
Unable to build constraint as '"Jones"' is invalid

Cheers
Wolfgang