Multiplication and addition are commutative binary operations, where the
equality between the operands has to be also checked in switched order.
By mistake, the operands were not compared with the other binary
operation, which is fixed by this commit.
The Tristate class (representing truth values that are either true,
false, or unknown) will be used at multiple ends. This moves it to a
separate header in order to reuse it properly.
This refactoring separates predicates from their declarations. The
purpose of this is to avoid duplicating properties specific to the
predicate declaration and not its occurrences in the program.
Patrick Lühne