Higher precision and generics

[sands@clipper.ens.fr (Duncan Sands)]

I am writing a package of matrix routines (in Ada!) in which some intermediate results
should be calculated in higher precision than normal.  The type for normal precision is
called Real and is a generic parameter of my package:
generic
   type Real is digits <>;
package Matrix_Stuff is
   ...
end;

In the body I would like to be able to declare Double to be a higher precision
floating point type, something like:
package body Matrix_Stuff is
   type Double is digits 2*Real'Digits;
   ...
end;

Unfortunately, this does not compile: apparently formal types like Real are not
considered static, so Real'Digits is not considered static, and only static
expressions are allowed in a "type X is digits expression" declaration.  (If
I write "type Double is digits 2*Float'Digits" then it compiles fine).

What can I do to get type Double to be of higher precision than Real?
Can anyone please help?  And does anyone know why formal types like Real are not
considered static?

Thanks a lot,

Duncan Sands.