/** This implements a "groupBy" function.   The function is applied to each
    member of list, returning a value y for each.  The results are grouped in a
    dictionary where the key is the value produced by the function and the
    value is an array of items from the list that returned that value y, in the
    order they were encountered in the list.
*/
groupBy[list, func] :=
{
   result = new dict
   for item = list
      result.addToList[func[item], item]

   return result
}

// println[groupBy[1 to 15, {|x| x mod 3}]]