fun killDuplicates "" = ""
  | killDuplicates s  = 
    let
	val c = String.sub(s, size s - 1)
    in
	if (isin'(String.substring(s, 0, size s - 1), c)) then
	    killDuplicates(String.substring(s, 0, size s -1))
	else
	    killDuplicates(String.substring(s, 0, size s -1)) ^ str(c)
    end;