Description: Define the class of all undirected simple hypergraphs. An undirected
simple hypergraph is a special (non-simple, multiple, multi-) hypergraph
for which the edge function e is an injective (one-to-one) function
into subsets of the set of vertices v , representing the (one or
more) vertices incident to the edge. This definition corresponds to the
definition of hypergraphs in section I.1 of Bollobas p. 7 (except that
the empty set seems to be allowed to be an "edge") or section 1.10 of
Diestel p. 27, where "E is a subset of [... the power set of V, that
is the set of all subsets of V" resp. "the elements of E are nonempty
subsets (of any cardinality) of V". (Contributed by AV, 19-Jan-2020)(Revised by AV, 8-Oct-2020)