Re: "accessibility check failed" on Node

[Shark8 <onewingedshark@gmail.com>]

At first blush, it looks like aNode is Null.
(position : constant NodeSets.Cursor := graph.nodes.Find (aNode.all'Access);)

Try adding "not null" to the parameter... or you could restructure it a bit:
    function findNode2
      (graph : DirectedGraph;
       aNode : access Node'Class)
      return  Node_Class_Access is

    begin
	declare
	    position : constant NodeSets.Cursor :=
	      graph.nodes.Find (aNode.all'Access);
	begin
	    if position /= NodeSets.No_Element then
		return Element (position);
	    else
		raise NoElement;
	    end if;
	exception
	    When PROGRAM_ERROR => raise NoElement;
	end;
    end findNode2;