automatic tag conversion?

[Stephan Heinemann <zombie@cs.tu-berlin.de>]

I would like to convert a Subtype of a tagged Supertype stored in a
variable of Supertype into a variable of Subtype. I do not want to
use the following:

if supervar'Tag = "Subtype" then
	polymorphic(Subtype(supervar));
elsif ...
else ...
end if;

Isn't there anything like:

t: Tag;
subvar: Subtype;
...
t := supervar'Tag;
subvar := t'convert(supervar); -- convert(t, supervar)
polymorphic(subvar);

?

It is because I would like to extend the Supertype and use dynamic
binding / polymorphism for the appropriate variable of Subtype without
having to do this String mess.

Thanks in advance,
Stephan