docs: def ketbra(i, j, Ne):
      """This function returns the outer product :math:`|i><j|` where\
   :math:`|i>` and :math:`|j>` are elements of the canonical basis of an\
   Ne-dimensional Hilbert space (in matrix form).
  
      >>> ketbra(2, 3, 3)
      Matrix([
      [0, 0, 0],
      [0, 0, 1],
      [0, 0, 0]])
  
      """
      return ket(i, Ne)*bra(j, Ne)