This is an implementation of a simple but efficient subgraph isomorphism algorithm.
query : (eq -> et -> Bool) -> (nq -> nt -> Bool) -> IGraph q eq nq -> IGraph t et nt -> Maybe (Vect q (Fin t))Using the given matching functions for node and edge labels, this
tries to align the `query` graph with the `target` graph in such
a way, that each query node is linked to a single target node.