A Matcher knows how to match itself against a concrete triple (the triple
to be matched) and a domain (of bindings to check and extend). It is part
of the improved-we-hope PatternStage code.
Answer true iff we match the triple t given the bindings
in d, updating those bindings if appropriate; the bindings
may be updated even if the match answers false.
Answer true iff we match the triple t given the bindings
in d, updating those bindings if appropriate; the bindings
may be updated even if the match answers false.