Magic_allow_disallow.Xval disallow_right : ('a, 'b, 'l * 'r) sided -> ('a, 'b, 'l * disallowed) sidedDisallows on the right.
val disallow_left : ('a, 'b, 'l * 'r) sided -> ('a, 'b, disallowed * 'r) sidedDisallows a the left.
Generalizes a right-hand-side allowed to be any allowance.