code: def add(self, addend_mat, axis=1):
          
          if self.finalized:
              if axis == 0:
                  raise NotImplementedError('The method is not yet implemented for the axis.')
              elif axis == 1:
                  for hid in xrange(self.shape[1]):
                      self.data[hid] = self.data[hid] + addend_mat
              elif axis == 2:
                  raise NotImplementedError('The method is not yet implemented for the axis.')
              else:
                  raise RuntimeError('The axis should be 0, 1, or 2.')
          else:
              raise RuntimeError('The original matrix must be finalized.')
docs: In-place addition
  
          :param addend_mat: A matrix to be added on the Sparse3DMatrix object
          :param axis: The dimension along the addend_mat is added
          :return: Nothing (as it performs in-place operations)