Module Weak: arrays of weak pointers
type 'a t;;

The type of arrays of weak pointers (weak arrays). A weak
pointer is an object that the garbage collector may erase at
any time.
A weak pointer is said to be full if it points to an object,
empty if the object was erased by the GC.
val create : int > 'a t;;

Weak.create n returns a new weak array of length n.
All the pointers are initially empty.
val length : 'a t > int;;

Weak.length ar returns the length (number of elements) of
ar.
val set : 'a t > int > 'a option > unit;;

Weak.set ar n (Some el) sets the nth cell of ar to be a
(full) pointer to el; Weak.set ar n None sets the nth
cell of ar to empty.
Raise Invalid_argument "Weak.set" if n is not in the range
0 to Weak.length a  1.
val get : 'a t > int > 'a option;;

Weak.get ar n returns None if the nth cell of ar is
empty, Some x (where x is the object) if it is full.
Raise Invalid_argument "Weak.get" if n is not in the range
0 to Weak.length a  1.
val fill: 'a t > int > int > 'a option > unit;;

Weak.fill ar ofs len el sets to el all pointers of ar from
ofs to ofs + len  1. Raise Invalid_argument "Weak.fill"
if ofs and len do not designate a valid subarray of a.
val blit : 'a t > int > 'a t > int > int > unit;;

Weak.blit ar1 off1 ar2 off2 len copies len weak pointers
from ar1 (starting at off1) to ar2 (starting at off2).
It works correctly even if ar1 and ar2 are the same.
Raise Invalid_argument "Weak.blit" if off1 and len do
not designate a valid subarray of ar1, or if off2 and len
do not designate a valid subarray of ar2.