(v2 (find-vertex graph value-2 error-if-not-found?)))
(or (and v1 v2 (find-edge-between-vertexes-if graph v1 v2 fn))
(when error-if-not-found?
(v2 (find-vertex graph value-2 error-if-not-found?)))
(or (and v1 v2 (find-edge-between-vertexes-if graph v1 v2 fn))
(when error-if-not-found?