datatype 'a arex =    ADD   of 'a binary  | SUB   of 'a binary  | MINUS of 'a unary  | TIP   of 'awithtype 'a binary = 'a arex * 'a arex     and 'a unary = 'a arex; val a1 = MINUS(ADD(TIP 1, SUB(TIP 2, TIP 5)));