Description: Define the class of the subgraph relation. A class s is a subgraph of a class g (thesupergraph of s ) if its vertices are also vertices of g , and its edges are also edges of g , connecting vertices of s only (see section I.1 in Bollobas p. 2 or section 1.1 in Diestel p. 4). The second condition is ensured by the requirement that the edge function of s is a restriction of the edge function of g having only vertices of s in its range. Note that the domains of the edge functions of the subgraph and the supergraph should be compatible. (Contributed by AV, 16-Nov-2020)