code: @Override
    public boolean add(E o) {
      if (map.containsKey(o)) {
        return false;
      } else {
        internalAdd(o);
        return true;
      }
    }
docs: Adds the specified element to this set if it is not already present.
  
  Remember that this set implementation uses == (not
  <code>equals()</code>) to test whether an element is present in the
  set.
  
  @param       o             element to add to this set
  @return      true          if the element was added,
  false         otherwise