The (partial) composition of two rules is mediated by the dependency graph E and the two matching morphisms e
. Since these are subgraph isomorphisms in our case, E i simply the union e1(R1) ∪ e2(L2). The (partial) match e1(R1) ∩ e2(L2) can be understood as a matching μ between R1 and L2, i.e., as a 1-1 relation of the matching nodes and edges. Whenever an edge is matched, then so are its incident vertices.