chap-2.3-forlan7e

datatype a =
    A1 of int
  | A2 of b * b
and      b =
    B1 of bool
  | B2 of a * a;
fun rev_a (A1 n)     = A1 n
  | rev_a (A2(x, y)) = A2(rev_b y, rev_b x)
and rev_b (B1 b)     = B1 b
  | rev_b (B2(x, y)) = B2(rev_a y, rev_a x);
val a = A2(B2(A1 3, A1 4), B1 true);
rev_a a;