Composing sequences (interesting to solve)

[porton@ex-code.com (Victor Porton)]

See an interesting and yet unsolved by me practical task to solve:

I have "elements" of various kinds (Base_Element'Class):

type Base_Element is null record;

There are many various types of element derived from
Base_Element. Some of these contain accesses to other
elements (which I allocate by "new"), so that this forms
trees.

To eliminate access types conversions I decided to limit
myself to use only Base_Element_Access (not accesses to
derived types) (BTW, Is it right design decision?):

type Base_Element_Access is access Base_Element'Class;

One of the types of elements is "sequence" (it is an ordered
container of elements):

type Sequence is new Base_Element with private;

I wish to produce sequences (allocated dynamically) by such
the syntax using overloaded function "&":

E1 & E2 & E3

The question is how to define function "&" so that it would
give the right result (access to one dynamically allocated
sequence) independingly on the number of elements?