This condition is signaled when a vertex can not be found in a graph.
| graph-error | This is the root condition for errors that occur while running code in CL-Graph.  | 
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| graph | The `graph` of which this edge is a part. Initargs::graph; Reader:graph. | 
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| vertex | The vertex or value that could not be found in the graph. Initargs::vertex; Reader:vertex. |