Search 'graph' for a vertex with element
'value'. The search is fast but inflexible because it uses an
associative-container. If you need more flexibity, see
search-for-vertex.
| find-vertex < cl-graph:basic-edge > < t > |
| find-vertex < cl-graph:basic-graph > < cl-graph:basic-vertex > |
| find-vertex < cl-graph:basic-graph > < t > |