Long comment make-two-ids-to-idx-map: A=B s/b B=A?

The comment explores whether predicate map nodes A=B and B=A
should be treated as the same map node.  This was not the case
in POPCO 1, nor is it implied by H&T1989.  Current conclusion:
No, they should be treated differently.
Moved A=B is B=A? comment to file in notes/
Fixed make-two-ids-to-idx-map to use seqs as keys.

Previous version used sets as keys, causing A=B and
B=A to be treated as the same.  This is not right.
See notes/ShouldReversedPredMapNodesBeTheSame for discussion.