Correct query for Multiplication

[sylvainhalle]

Currently, the explanation for a multiplication is such that the output value always depends on all its arguments (default behavior since it inherits from AtomicFunction). This is not the case if one of the arguments is 0, in which case it alone explains the result.

The production of a value for multiplication should be modified to work in the same way as, e.g. BooleanAnd when it is given a false argument.