code: @Override
      public boolean addAll(Collection<? extends Pair<K, V>> c) {
          return backedSet.addAll(c);
      }
docs: Adds all of the elements in the specified collection to this applyTransformToDestination if
  they're not already present (optional operation).  If the specified
  collection is also a applyTransformToDestination, the <tt>addAll</tt> operation effectively
  modifies this applyTransformToDestination so that its value is the <i>union</i> of the two
  sets.  The behavior of this operation is undefined if the specified
  collection is modified while the operation is in progress.
  
  @param c collection containing elements to be added to this applyTransformToDestination
  @return <tt>true</tt> if this applyTransformToDestination changed as a result of the call
  @throws UnsupportedOperationException if the <tt>addAll</tt> operation
  is not supported by this applyTransformToDestination
  @throws ClassCastException            if the class of an element of the
  specified collection prevents it from being added to this applyTransformToDestination
  @throws NullPointerException          if the specified collection contains one
  or more null elements and this applyTransformToDestination does not permit null
  elements, or if the specified collection is null
  @throws IllegalArgumentException      if some property of an element of the
  specified collection prevents it from being added to this applyTransformToDestination
  @see #add(Object)