generic
   -- defines maximum depth of tree
   type Node_Level is (<>);
package Tree is

   -- it's a binary tree. for now.
   type Tree_Direction is (Left, Right);

   type Direction_Array is array (Node_Level range <>) of Tree_Direction;

   type Path_To_Node (Max : Node_Level) is record
      Directions : Direction_Array (Node_Level'First .. Max);
   end record;

   -- .. somewhere here, the actual Tree structure is defined
end Tree;