Edge Contraction
The collapse selected option of
the Edit menu replaces all selected
items with a single ``super vertex'' sv.
Then (u,sv) is an edge if and only if
u was not selected, and there exists a
vertex v in the subgraph represented by sv
such that (u, v) was an edge. A similar
argument holds for (sv, u).
In particular, an edge is contracted by
selecting its endpoints with <1st>, then
selecting the Collapse Subgraph option
or typing s from the keyboard.
Super vertices are replaced with the subgraphs
they represent by selecting them, then
selecting the expand-selected option
from the Edit menu or typing S.