The role of isomorphism in synthetic analysis. Pruning the search tree by finding disjoint isomorphic substructuresElectronic supplementary information (ESI) available: Fig. S1, all possible 9-atom substructures of the steroid skeleton, and Fig. S2, all possible pairs of disjoint isomorphic steroid substructures with 9 atoms. See http://www.rsc.org/suppdata/cc/b3/b300935a

Isomorphism is an equivalence relation that is less stringent than identity (equality), and it is useful for synthetic analysis, since it allows one to find reflexive routes for targets even when they do not have an element of symmetry.