module alists.
  
  %%% lookup is like LISP assoc; it returns the value, if any, for a key in
  %%% an alist.  There is a value if the key is a member of some Key-Value pair
  %%% in the alist.
  lookup Key (acons Key Value Alist) Value.
  lookup Key (acons K V Alist) Value :- lookup Key Alist Value.