In a similar vein, it is easy to grow chains, e.g. by crossover of an H at the end of a chain with CH 4 , if an extension by one carbon unit is desired, and to enlarge or shrink rings by cuts of size 2 and a subsequent join using two rings as parents. If the initial set of graphs G contains CH 4 , NH 3 , H 2 O, and H 2 , we can in particular build all cycle-free molecules even if only 2-cuts of size 1 are allowed. If only 2-cuts are allowed, however, cycles have to be present at the outset to allow the formation of cycles or additional bridges, since 2-cuts in cycle-free graphs necessarily have size 1. The closure of a new ring, however, requires that the join edges connected two vertices on one fragment with a vertex with degree-defect 2 on the other fragment, or two pairs of vertices.