CrossProduct of relations with overlapping schema

[ogrady]

Creating the CrossProduct of two relations with overlapping schema

schema(A) = ["a","b"]
schema(B) = ["b","c"]

Leads to an error (tuples don't match schema). This stems from both schemas having a field b which, which can't be reflected in the schema, which internally is a key-set. So it should either be renamed to something like b' automatically or fail with a more appropriate error message.

NB: formally, this operation is not defined anyway. Having to rename it is the most appropriate way to handle it.