Optional body for nested generic

[Simon Wright <simon@pushface.org>]

I was surprised to find that GNAT makes it optional to have a body for a
nested generic package (in the code below, gpack1 has an empty body,
gpack2 doesn't have a body at all).

package Pack is
   pragma Elaborate_Body;
   generic
   package Gpack1 is
      procedure Proc;
      pragma Import (C, Proc, "proc");
   end Gpack1;
   generic
   package Gpack2 is
      procedure Proc;
      pragma Import (C, Proc, "proc");
   end Gpack2;
end Pack;

package body Pack is
   package body Gpack1 is
   end Gpack1;
end Pack;

I suppose that the reasoning for disallowing unrequired bodies (roughly,
making it impossible to obsolete the ada library by adding/removing
something with no other impact) only applies at library level, where
there's a correspondence to a file?