Furthermore, all other frontier graph fragments can be created by composing 2 or more minimal graph fragments, as shown in Figure 8.
the entire set of frontier graph fragments (and all rules derivable from these fragments) can be computed systematically as follows: compute the set of minimal frontier graph fragments, compute the set of graph fragments resulting from composing 2 minimal frontier graph fragments, compute the set of graph fragments resulting from composing 3 minimal graph fragments, etc.