Example of internal-external overlap between subnets. Subnets A and B, connected by a common product of reaction R1. Metabolite nodes are shown as squares and the reaction node as an octagon. (a) Bipartite representation of the network (b) Reduced metabolites only network. In (b) subnets are fully separated by making c external, but in (a) reaction R1 is included in subnet A to ensure full representation of the network environment of internal node a .Hence node b acts as an external for subnet A, but as internal for subnet B.