(:documentation "Takes a GRAPH and a TEST-FN (a single argument function
returning NIL or non-NIL), and filters the graph nodes according to
the test-fn (those that return non-NIL are accepted), returning
(:documentation "Takes a GRAPH and a TEST-FN (a single argument function
returning NIL or non-NIL), and filters the graph nodes according to
the test-fn (those that return non-NIL are accepted), returning