Assuming that the value-or-vertex corresponds to
one of the vertexes for edge, this method returns the other vertex
of edge. If the value-or-vertex is not part of edge, then an error
is signaled. [?? Should create a new condition for this]
| other-vertex < cl-graph:graph-container-edge > < cl-graph:graph-container-vertex > |
| other-vertex < cl-graph:graph-container-edge > < t > |