Non-standard functions in GNAT's Ada.Containers packages?

["G.B." <bauhaus@notmyhomepage.invalid>]

In Ada.Containers.{Kind}s of GCC 11.2, I find functions defined
by the pattern:

   function Empty (Capacity : Count_Type := 1000) return {Kind};

I couldn't find these functions in the LRM's package specifications.

Upon type derivation, then, these functions appear to require
non-portable source text when using GNAT's standard Ada
containers.  For example,

     package Real_Vectors is new Ada.Containers.Vectors (...);
     type Fancy_Vectors is new Real_Vectors.Vector with private;

     18.     type Fancy_Vectors is new Real_Vectors.Vector with record
                  |
         >>> type must be declared abstract or "Empty" overridden
         >>> "Empty" has been inherited at line 14
         >>> "Empty" has been inherited from subprogram at a-convec.ads:125, instance at line 8


While composition would not incur this effect, etc. etc., still
the resulting source text would need to override a "standard" function,
but this function, Empty, might not exist at all when using other
standard Ada compilers.

Am I missing something?