### MC44 Unassembled ﬁnite-element matrix: generate the element or supervariable
connectivity graph

Given the structure of an unassembled ﬁnite-element matrix, this subroutine groups
the variables into supervariables and optionally generates either the element
connectivity graph or the supervariable connectivity graph.

A supervariable is a collection of one or more variables, such that each variable
belongs to the same set of ﬁnite elements. In the supervariable connectivity graph,
the nodes are the supervariables and the edges are constructed by making the
supervariables of each ﬁnite element pairwise adjacent. The supervariable
connectivity graph, together with the number of variables in each supervariable,
provide a compact representation of the variable connectivity graph. In the element
connectivity graph, the nodes are the elements and the edges are constructed by
deﬁning two elements to be adjacent whenever they have one or more variables in
common.