When explicitly calling a CLIPS function or operator using
the Call() function, an exception arises on correct calls
while bad calls just return wrong values: this is due to a
misinterpretation of the return value of CLIPS
FunctionCall() API that returns 0 on success and
nonzero on failure. Also, Call() still suffers the same
issue that affected Eval(), as it does not clear the error
flag from the execution environment.
Both issues should be fixed by next CVS release.