Make XSB usage of operators less naive

[Joachim Jansen]

for now, an expression x is y op z is translated into an XSB call:

type(Y), type(Z), X is Y op Z, type(X).

This is however not always necessary (e.g. if X and Y is given, Z can often be derived by rewriting the expression).

Attached such an example.

Related to issue #670