extern fun add (x: int, y: int): int

val _ = add (1, 1)
val _ = 1 \add 1 // '\' turns this occurrence of [add] to be infix

infix (+) add // [add] becomes an infix operator
val _ = 1 add 1
val _ = op add (1, 1) // 'op' turns this occurrence of [add] to be nonfix

nonfix add // [add] becomes a nonfix operator
val _ = add (1, 1)
val _ = 1 \add 1