A SmallGraphMem is a memory-based Graph suitable only for Small models
(a few triples, perhaps a few tens of triples), because it does no indexing,
but it stores onlya single flat set of triples and so is memory-cheap.
author: kers
Answer true iff t matches some triple in the graph. If t is concrete, we
can use a simple membership test; otherwise we resort to the generic
method using find.