A vertex v is incident with an edge e if v 2 e; then e is an edge at v. The two vertices incident with an edge are its endvertices or ends, and
an edge joins its ends. An edge f x; y g is usually written as xy (or yx).
If x 2 X and y 2 Y , then xy is an X{Y edge. The set of all X{Y edges
in a set E is denoted by E(X; Y ); instead of E(f x g; Y ) and E(X; f y g)
we simply write E(x; Y ) and E(X; y). The set of all the edges in E at a
vertex v is denoted by E(v).