docs: def define_rho_vector(rho, Ne):
      u"""Define the vectorized density matrix.
  
      >>> from sympy import pprint
      >>> rho = define_density_matrix(3)
      >>> pprint(define_rho_vector(rho, 3), use_unicode=True)
      ⎡  ρ₂₂  ⎤
      ⎢       ⎥
      ⎢  ρ₃₃  ⎥
      ⎢       ⎥
      ⎢re(ρ₂₁)⎥
      ⎢       ⎥
      ⎢re(ρ₃₁)⎥
      ⎢       ⎥
      ⎢re(ρ₃₂)⎥
      ⎢       ⎥
      ⎢im(ρ₂₁)⎥
      ⎢       ⎥
      ⎢im(ρ₃₁)⎥
      ⎢       ⎥
      ⎣im(ρ₃₂)⎦
  
      """
      rho_vect = []
      for mu in range(1, Ne**2):
          i, j, s = IJ(mu, Ne)
          i = i-1; j = j-1
          rho_vect += [part_symbolic(rho[i, j], s)]
      return Matrix(rho_vect)