Priority rules

[Noud Aldenhoven]

Dear Prolog mailing list,

Is there a way to give "priority" to rules in (gnu)prolog?

For example I have four persons A, B, C and D and I have to build a schedule for them. But person A and person B don't like each other and prefer not to work with each other. Hence I would like to have a rule

better_not_together(A,B)

This rule should be true if there is a possibility to make it true. But if there is no such solution it should become false. Therefore the priority of this rule of becoming true should be lower than the priority of the other rules.

Can this be done in prolog? Or maybe an equivalent question is, does prolog support multi-valued logic? (values can be either true (1), false (0) or some value between 0 and 1)

Greetings,

Noud