(*  This program returns the nth fibonacci number            *
 *  or 0 if the parameter is invalid.                        *
 *  Based on a program by Jonathan Paisley and Matt Wemmie.  *)

function main ( elementWanted : integer ) : integer
  if elementWanted < 1 then
     0
  else
     addNext(1, elementWanted, 0, 1)

function addNext(currentElement : integer,
                 elementWanted  : integer,
                 previousSum    : integer,
                 currentSum     : integer  ) : integer
  if elementWanted = currentElement then
     currentSum
  else
     addNext( currentElement + 1,
              elementWanted,
              currentSum,
              previousSum + currentSum )