Module type Piecewise_linear_intf.S_invertible

module type S_invertible = sig .. end

include Piecewise_linear_intf.S
create enforces that the x (key) values are strictly increasing. It also enforces that the y (value) values are either strictly increasing or strictly decreasing. These two properties give us invertibility.
val get_inverse : t -> value -> key
get_inverse t value is the inverse operation of get t key.