title: how to merge an attribute in multi-inheritence
body: <p>Repeatedly inheriting from 2 classes having the same parent, I fall into the classic case of inheriting 2 times of the same attribute. I'd like to merge the 2 attributes into one and tried to do it with an undefine, but it gets me a compile error.
  The other solution I see is renaming the attribute from one of both parents, but as I understand each instance of my D class would have an useless attribute which is not what I want...</p>
  
  <p><code>Error: Undefine subclause lists name of frozen feature or attribute or 
      C external.
  What to do: unless you can change the status of the feature in the parent,
      remove its name from Undefine subclause since it cannot be undefined.</code> </p>
  
  <p>How to merge 2 attributes from repeatedly inherited classes</p>
  
  <pre><code>class A
      serial: STRING
  
  end -- class A
  
  class B
  
  inherit
      A
  
  end -- class B
  
  
  class C
  
  inherit
      A
  
  end -- class C
  
  
  class D
  
  inherit
      B
          undefine 
              serial -- error seems to appear here in that case
          end
      C
  
  end -- class D
  </code></pre>