How do you specialize a generic function from a nested function?

[EGarrulo <egarrulo@gmail.com>]

I am trying to write a generic predicate that negates a supplied predicate.  The generic predicate compiles, but the compilation of client code fails with "generic subprogram cannot be called" (because the client code supplies a nested predicate, I suppose).  Here is the code (see the comment in the last file).

------------- negate.ads ------------------

generic
   type Value_Type is private;
   with function Predicate (Value : Value_Type) return Boolean;
   -- I have also tried with:
   -- Predicate : access function (Value : Value_Type) return Boolean;
function Negate (Value : Value_Type) return Boolean;

------------- negate.adb ------------------

function Negate (Value : Value_Type) return Boolean is
begin
   return not Predicate (Value);
end Negate;

------------- test.adb ------------------

with Negate;

procedure test is
   function Is_Zero (Value: Integer) return Boolean is (Value = 0);

   Is_Not_Zero : access function (Value: Integer) return Boolean 
   := Negate(Integer, Is_Zero'Access); -- <<== ##### ERROR HERE #####
begin
   null;
end test;

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What is wrong?  And -- in any case -- could the code be simplified?  Thank you.