Added v1 of match-propn-pairs, match-propn-pair-components, match-args.
Not right: Doesn't merge dupe propn pairings, and add weight (which
I thought could be done later, but maybe needs to happen here).
Also won't allow sets of identical predicate keywords.  Probably won't
allow them if I make Pred records, either, though.  Maybe set isn't
quite the right representation.