# Module type Binary_searchable_intf.Indexable

    module type Indexable = sig .. end

An `Indexable` type is a finite sequence of elements indexed by consecutive integers `0` ... `length t - 1`. `get` and `length` must be O(1) for the resulting `binary_search` to be lg(n).

    type elt

    type t

    val get : t ->
           int -> elt

    val length : t -> int

    module For_test: sig .. end

To implement the test provided by `Binary_searchable`, we need to be able to construct `t` with two different values `small < big`.