docs: def vector_element(r, i, j):
      r"""Extract an matrix element of a vector operator.
  
      >>> r = define_r_components(2)
      >>> vector_element(r, 1, 0)
      Matrix([
      [x_{21}],
      [y_{21}],
      [z_{21}]])
  
      """
      return Matrix([r[p][i, j] for p in range(3)])