Predicate signature InputSig::compatibleTypes

Holds if t1 and t2 are compatible types.

This predicate must be symmetric and reflexive.

This predicate is used in the following way: If the data flow library tracks an object from node n1 to n2 using solely value-preserving steps, then it will check that the types of n1 and n2 are compatible. If they are not, then flow will be blocked.