code: public static void putAt(MutableComboBoxModel self, int index, Object i) {
          self.insertElementAt(i, index);
      }
docs: Allow MutableComboBoxModel to work with subscript operators.<p>
  <b>WARNING:</b> this operation does not replace the item at the
  specified index, rather it inserts the item at that index, thus
  increasing the size of the model by 1.
  
  @param self  a MutableComboBoxModel
  @param index an index
  @param i     the item to insert at the given index
  @since 1.6.4