Batteries user guide : BatNumber.Discrete

`sig
type discrete
val to_int : BatNumber.Discrete.discrete -> int
val succ : BatNumber.Discrete.discrete -> BatNumber.Discrete.discrete
val pred : BatNumber.Discrete.discrete -> BatNumber.Discrete.discrete
val ( -- ) :
BatNumber.Discrete.discrete ->
BatNumber.Discrete.discrete -> BatNumber.Discrete.discrete BatEnum.t
val ( --- ) :
BatNumber.Discrete.discrete ->
BatNumber.Discrete.discrete -> BatNumber.Discrete.discrete BatEnum.t
end`