"accessibility check failed" on Node

[Thomas Schmidt <TC.Schmidt@gmx.net>]

Hi,

I'm new to Ada. I'm trying to write some kind of neural net software. 
Therefore I've written a small library to implement the neural graph. 
All nodes are collected in a Hashed_Set. Neurons are specialized Nodes 
of the graph. To lookup a given Neuron/Node I need the following 
subprogram "findNode".

Now what I don't understand: Why does "findNode1" work well while 
"findNode2" raises PROGRAM_ERROR at the line indicated below?


Given the following declarations:

   -- Node of a directed graph
   type Node is new NodeBase with private;	-- NodeBase is tagged.
   type Node_Class_Access is access all Node'Class;

and the DirectedGraph is a tagged record with one of its elements 
(named "nodes") being a NodeSet defined as a Set of my NodeSets package

  package NodeSets is new Ada.Containers.Hashed_Sets (
     Element_Type        => Node_Class_Access,
     Hash                => Nodes.hash,
     Equivalent_Elements => Nodes.equiv);

   --------------
   -- findNode1 --
   --------------

   function findNode1
     (graph : DirectedGraph;
      aNode : access Node'Class)
      return  Node_Class_Access is

      position : constant NodeSets.Cursor :=
        graph.nodes.Find (Node (aNode.all)'Access);	-- <<< Works well

   begin
      if position /= NodeSets.No_Element then
         return Element (position);
      else
         raise NoElement;
      end if;
   end findNode1;

   --------------
   -- findNode2 --
   --------------

   function findNode2
     (graph : DirectedGraph;
      aNode : access Node'Class)
      return  Node_Class_Access is

      position : constant NodeSets.Cursor :=
        graph.nodes.Find (aNode.all'Access);	-- <<< Raises 
PROGRAM_ERROR with "accessibility check failed"

   begin
      if position /= NodeSets.No_Element then
         return Element (position);
      else
         raise NoElement;
      end if;
   end findNode2;

"findNote" will be called with a Neuron as its second argument.


Many thanks for your explanations

Thomas