How can I create an empty array of a generic type?

[b.mcguinness747@gmail.com]

I have a generic package


generic
  type Element_Type is private;
  type Key_Component_Type is private;
  with function "<" (Left, Right : Key_Component_Type) return Boolean is <>;
  with function "=" (Left, Right : Element_Type) return Boolean is <>;
package Ternary_Trees is

...


private

...

  Empty_Key  : constant Key_Type(1..0) := (others => Key_Component_Type'First);

...

end Ternary_Trees;


When I try to compile the package, I get an error message:

ternary_trees.ads:77:54: prefix for "First" attribute may not be private type


How can I make this work?  I want to create an empty array so I can build up a key through concatenation as I descend recursively through a tree.

Thanks.

--- Brian McGuinness