    datatype Colour = R | B

    datatype 'a RBtree = E | N of Colour * 'a * 'a RBtree * 'a RBtree

    (* Dieses lookup funktioniert nur fuer Elemente vom Typ int *)

    fun lookup (x,E) = false
      | lookup (x,N(_,y,l,r)) =
           if x < y then lookup(x,l)
           else if y < x then lookup(x,r)
           else true

    fun balance (B,x,N(R,y,N(R,z,t1,t2),t3),t4) =
           N(R,y,N(B,z,t1,t2),N(B,x,t3,t4))
      | balance (B,x,N(R,y,t1,N(R,z,t2,t3)),t4) =
           N(R,z,N(B,y,t1,t2),N(B,x,t3,t4))
      | balance (B,x,t1,N(R,y,N(R,z,t2,t3),t4)) =
           N(R,z,N(B,x,t1,t2),N(B,y,t3,t4))
      | balance (B,x,t1,N(R,y,t2,N(R,z,t3,t4))) =
           N(R,y,N(B,x,t1,t2),N(B,z,t3,t4))
      | balance t = N t

    fun insert(x,t) =
       let
          fun ins E = N(R,x,E,E)
            | ins (t as N(c,y,l,r)) =
                 if x < y then balance (c,y,ins l,r)
                 else if y < x then balance (c,y,l,ins r)
                 else t
          val N(_,y,l,r) = ins t
       in N(B,y,l,r)
       end