def insert_vector(indexes, val)
      new_index = @index.add(*indexes)
      # May be create +=
      (new_index.size - @index.size).times { @data << val }
      @index = new_index
    end
def insert_vector(indexes, val)
      new_index = @index.add(*indexes)
      # May be create +=
      (new_index.size - @index.size).times { @data << val }
      @index = new_index
    end
Helper method for []=.
 Add a new index and assign it value