`module Queue: sig .. end`

First-in first-out queues.

This module implements queues (FIFOs), with in-place modification.

**Warning** This module is not thread-safe: each `Queue.t`

value
must be protected from concurrent access (e.g. with a `Mutex.t`

).
Failure to do so can lead to a crash.

```
type t('a);
```

The type of queues containing elements of type

`'a`

.```
exception Empty;
```

```
let create: unit => t('a);
```

Return a new queue, initially empty.

```
let add: ('a, t('a)) => unit;
```

`add x q`

adds the element `x`

at the end of the queue `q`

.```
let push: ('a, t('a)) => unit;
```

`push`

is a synonym for `add`

.```
let take: t('a) => 'a;
```

`take q`

removes and returns the first element in queue `q`

,
or raises `Empty`

if the queue is empty.```
let pop: t('a) => 'a;
```

`pop`

is a synonym for `take`

.```
let peek: t('a) => 'a;
```

`peek q`

returns the first element in queue `q`

, without removing
it from the queue, or raises `Empty`

if the queue is empty.```
let top: t('a) => 'a;
```

`top`

is a synonym for `peek`

.```
let clear: t('a) => unit;
```

Discard all elements from a queue.

```
let copy: t('a) => t('a);
```

Return a copy of the given queue.

```
let is_empty: t('a) => bool;
```

Return

`true`

if the given queue is empty, `false`

otherwise.```
let length: t('a) => int;
```

Return the number of elements in a queue.

```
let iter: ('a => unit, t('a)) => unit;
```

`iter f q`

applies `f`

in turn to all elements of `q`

,
from the least recently entered to the most recently entered.
The queue itself is unchanged.```
let fold: (('b, 'a) => 'b, 'b, t('a)) => 'b;
```

`fold f accu q`

is equivalent to `List.fold_left f accu l`

,
where `l`

is the list of `q`

's elements. The queue remains
unchanged.```
let transfer: (t('a), t('a)) => unit;
```

`transfer q1 q2`

adds all of `q1`

's elements at the end of
the queue `q2`

, then clears `q1`

. It is equivalent to the
sequence `iter (fun x -> add x q2) q1; clear q1`

, but runs
in constant time.