datatype 'a num_dict
  = NumD of {plus : 'a * 'a -> 'a, neg : 'a -> 'a}

fun sub (NumD {plus, neg}) (m, n) = plus (m, neg n)

val x = sub (NumD {plus=op+, neg=op ~}) (0, 1)
val y = sub (NumD {plus=op+, neg=op ~}) (0.0, 1.0)