This patch adds arity checking to relations, facts and retractions so that if a user specifies that a rel "foo" shall be 3-tuple in the def-rel form it will be illegal to add a 4-tuple as a fact to the db. This protects the state of the DB against a class of user errors which would otherwise silently fail through.
This is more a work proposal than anything, as it may well be legitimate to have a variable arity relation. I would just consider such a construct suspect, and this patch supports the way I've approached pldb.
