In the contraction Ω/"S" by an arbitrary edge set "S", the edge set is "E" − "S". (We let "G" = ("V", "E").) The vertex set is the class of vertex sets of balanced components of the subgraph ("V", "S") of Ω. That is, if ("V", "S") has balanced components with vertex sets "V"1, ..., "V""k", then Ω/"S" has "k" vertices "V"1, ..., "V""k" . An edge "e" of Ω, not in "S", becomes an edge of Ω/"S" and each endpoint "v""i" of "e" in Ω that belongs to some "Vi" becomes the endpoint "Vi" of "e" in Ω/"S" ; thus, an endpoint of "e" that is not in a balanced component of ("V", "S") disappears. An edge with all endpoints in unbalanced components of ("V", "S") becomes a loose edge in the contraction. An edge with only one endpoint in a balanced component of ("V", "S") becomes a half-edge. An edge with two endpoints that belong to different balanced components becomes a link, and an edge with two endpoints that belong to the same balanced component becomes a loop.