anchor.diff:    (assert-type! to   vertex)
   
     (when (/= (vertex-graph from) (vertex-graph to))
  -    (error! "These nodes are on different graphs"))
  +    (format 1 "(add-edge! {#from} {#to}): vertex value cannot be nil"))
   
     (.<! (vertex-out-edges from) to   true)
     (.<! (vertex-in-edges  to)   from true)
  
positive.code:    «vertex: \"a\"»
     out = nil
     ```"
    (with (visited {})
      (loop [(node root)] []
        (unless (.> visited node)
  
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 '() {}))))
  
  