The Louie-Kercel representation of ( M,R ). Lines with open arrowheads are metabolic productive reactions, for instance A produces B. Lines with filled arrowheads represent catalysis. Conventionally these point to the substrate of a metabolic reaction. For instance f catalyses the production of B from A, and this is indicated by f pointing to A. Likewise, Φ catalyses the production of f from B, so an open-headed arrow goes from B to f, and a filled arrow from Φ to B.