anchor.diff:  (defun union (xs ys)
     "Set-like union of XS and YS.
   
  -   Example:
  +   ### Example:
      ```
      > (union '(1 2 3 4) '(1 2 3 4 5))
      out = (1 2 3 4 5)
  
positive.code:   (assert-type! f function)
    (assert-type! ac function)
    (foldl ac z (map f xs)))
  
  ;; AUTOMATICALLY GENERATED
  ;; DO NOT EDIT please.
  
negative.code: (defun \\ (xs ys)
    "The difference between XS and YS (non-associative.)
  
     ### Example:
     ```
     > (\\ '(1 2 3) '(1 3 5 7))
     out = (2)
  