anchor.diff:  (defun \\ (xs ys)
     "The difference between XS and YS (non-associative.)
   
  -   Example:
  +   ### Example:
      ```
      > (\\ '(1 2 3) '(1 3 5 7))
      out = (2)
  
positive.code: (defun union (xs ys)
    "Set-like union of XS and YS.
  
     Example:
     ```
     > (union '(1 2 3 4) '(1 2 3 4 5))
     out = (1 2 3 4 5)
  
negative.code: (defun nub (xs)
    "Remove duplicate elements from XS.
  
     ### Example:
     ```
     > (nub '(1 1 2 2 3 3))
     out = (1 2 3)
  