%% ---
%%  Excerpted from "Seven Languages in Seven Weeks",
%%  published by The Pragmatic Bookshelf.
%%  Copyrights apply to this code. It may not be used to create training material, 
%%  courses, books, articles, and the like. Contact us if you are in doubt.
%%  We make no guarantees that this code is fit for any purpose. 
%%  Visit http://www.pragmaticprogrammer.com/titles/btlang for more book information.
%%---
concatenate([], List, List).
concatenate([Head|[]], List, [Head|List]). 
concatenate([Head1|[Head2|[]]], List, [Head1, Head2|List]).
concatenate([Head1|[Head2|[Head3|[]]]], List, [Head1, Head2, Head3|List]).