# 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.

It also enforces certain finiteness conditions: the x and y values must be finite (non-nan, and non-infinite), and differences of consecutive x values and consecutive y values must be finite.

(Conceivably, one might have a case where one wants to loosen the conditions on x and y values to non-strict monotonicity, so that one does not have true invertibility, but only a sort of formal invertibility. If that use case arises, a separate functor like `Make_formally_invertible` could be added, so that `Make_invertible` maintains its stricter semantics.)

    val get_inverse : t -> value -> key

`get_inverse t value` is the inverse operation of `get t key`.