Search 'graph' for a vertex with element
   'value'. The 'key' function is applied to each element before that
   element is compared with the value. The comparison is done using the
   function 'test'. If you don't need to use key or test, then consider
   using find-vertex instead. 
| search-for-vertex < cl-graph:basic-graph > < cl-graph:basic-vertex > | 
| search-for-vertex < cl-graph:basic-graph > < t > |