Using Access Types for a simple solution

["ldries46" <bertus.dries@planet.nl>]

Let me first explain the problem. I do have a matrix of records

type Box is record
   ...
   ...
end record

Grid : array( 1 .. N,  1 .. M) of Box;

Now I must do something with a random set out of elements from the matrix. I 
would like to create an array of pointers to the elements of the matrix so 
that I can use these elements and file the changes directly if the array. So 
I created a type:

type Box_Access is access all Box;

and make an array

Box_Array : array(1 .. Q) of Box_Acess;

When try to use that in a simple statement

Box_Array(n) := Grid(n1, n2)'Access;

I get the error (only for this statement): prefix of 'Access attribute must 
be aliased.

I tried:

Grid : aliased array( 1 .. N,  1 .. M) of Box;

But that also gives the same error.
I cannot imagine that what I want is not possible in Ada as I can realise it 
in C++ so I do miss something.

L. Dries