module For_test:sig..end
Binary_searchable, we need to be able to
construct t with two different values small < big. We also need to be able to
build a t from an elt array.val small : Binary_searchable_intf.Indexable.elt
val big : Binary_searchable_intf.Indexable.elt
val of_array : Binary_searchable_intf.Indexable.elt array ->
Binary_searchable_intf.Indexable.t