anchor.diff:     «vertex: \"a\"»
      out = nil
      ```"
  +  (assert-type! root vertex)
     (with (visited {})
       (loop [(node root)] []
         (unless (.> visited node)
  
positive.code: (defun add-vertex! (graph value)
    "Create a vertex with the corresponding VALUE and add it to the GRAPH."
    (assert-type! graph graph)
    (when (.> graph :vertex-lookup value)
      (error! "value is already in this graph"))
  
    (with (vertex (make-vertex value graph {} {}))
      (push-cdr! (graph-vertices graph) vertex)
  
negative.code:  super-vertices, and other nifty features."
  
  (import data/struct ())
  (import data/format ())
  
  (defstruct graph
    "Create a new, empty graph."
    (fields
      (immutable (hide vertices))
      (immutable (hide vertex-lookup)))
    (constructor new
      (lambda () (new '() {}))))
  
  