19.24 
Module Queue: firstin firstout queues 

This module implements queues (FIFOs), with inplace modification.
type 'a t
The type of queues containing elements of type 'a
.
exception Empty
Raised when take
is applied to an empty queue.
val create: unit > 'a t
Return a new queue, initially empty.
val add: 'a > 'a t > unit
add x q
adds the element x
at the end of the queue q
.
val take: 'a t > 'a
take q
removes and returns the first element in queue q
,
or raises Empty
if the queue is empty.
val peek: 'a t > 'a
peek q
returns the first element in queue q
, without removing
it from the queue, or raises Empty
if the queue is empty.
val clear : 'a t > unit
Discard all elements from a queue.
val length: 'a t > int
Return the number of elements in a queue.
val iter: f:('a > unit) > 'a t > 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.