Add third and fourth overlay to hierarchical overview image

Author: frank-at-adacore

courses/fundamentals_of_ada/130_program_structure/04-hierarchical_library_units.rst

@@ -52,6 +52,18 @@ Visibility Across a Hierarchy
       :width: 70%
       :align: center
 
+.. container:: overlay 3
+
+   .. image:: hierarchical_visibility_3.svg
+      :width: 70%
+      :align: center
+
+.. container:: overlay 4
+
+   .. image:: hierarchical_visibility_4.svg
+      :width: 70%
+      :align: center
+
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 Programming by Extension
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