`type +'a period constraint 'a = [< date_field ]`
`type t = date_field period`

Type of a period.

#### Period is an additive monoid

`val empty : 'a period`

The empty period.

`val add : 'a period -> 'a period -> 'a period`

Addition of periods.

`val sub : 'a period -> 'a period -> 'a period`

Substraction of periods.

`val opp : 'a period -> 'a period`

Opposite of a period.

#### Periods are comparable

`val equal : 'a period -> 'b period -> bool`

Equality function between two periods.

- see Utils.Comparable.html#VALequal

  Utils.Comparable.equal

- since 1.09.0

`val compare : 'a period -> 'b period -> int`

Comparison function between two periods.

- see Utils.Comparable.html#VALcompare

  Utils.Comparable.compare

`val hash : 'a period -> int`

Hash function for periods.

- see Utils.Comparable.html#VALhash

  Utils.Comparable.hash

- since 2.0