old_contents: def YilmIndexVector(i, l, m):
      """
      Compute the index of an 1D array of spherical harmonic coefficients
      corresponding to i, l, and m.
  
      Usage
      -----
      index = YilmIndexVector (i, l, m)
  
      Returns
      -------
      index : integer
          Index of an 1D array of spherical harmonic coefficients corresponding
          to i, l, and m.
  
      Parameters
      ----------
      i : integer
          1 corresponds to the cosine coefficient cilm[0,:,:], and 2 corresponds
          to the sine coefficient cilm[1,:,:].
      l : integer
          The spherical harmonic degree.
      m : integer
          The angular order.
  
      Notes
      -----
      YilmIndexVector will calculate the index of a 1D vector of spherical
      harmonic coefficients corresponding to degree l, angular order m and i
      (1 = cosine, 2 = sine). The index is given by l**2+(i-1)*l+m.
      """
  
      return l**2 + (i - 1) * l + m
  
new_contents: def YilmIndexVector(i, l, m):
      """
      Compute the index of a 1D array of spherical harmonic coefficients
      corresponding to i, l, and m.
  
      Usage
      -----
      index = YilmIndexVector (i, l, m)
  
      Returns
      -------
      index : integer
          Index of a 1D array of spherical harmonic coefficients corresponding
          to i, l, and m.
  
      Parameters
      ----------
      i : integer
          1 corresponds to the cosine coefficient Ylm = cilm[0,:,:], and 2
          corresponds to the sine coefficient Yl,-m = cilm[1,:,:].
      l : integer
          The spherical harmonic degree.
      m : integer
          The angular order, which must be greater or equal to zero.
  
      Notes
      -----
      YilmIndexVector will calculate the index of a 1D vector of spherical
      harmonic coefficients corresponding to degree l, (positive) angular order
      m and i (1 = cosine, 2 = sine). The index is given by l**2+(i-1)*l+m.
      """
      if l < 0:
          raise ValueError('The spherical harmonic degree must be positive. '
                           'Input value is {:s}'.format(repr(l)))
      if m < 0:
          raise ValueError('The angular order must be positive. '
                           'Input value is {:s}'.format(repr(m)))
      if m >= l:
          raise ValueError('The angular order must be less than or equal to '
                           'the spherical harmonic degree. Input degree is {:s}.'
                           ' Input order is {:s}.'.format(repr(l), repr(m)))
  
      return l**2 + (i - 1) * l + m
  
subject: Add error checks to YilmIndexVector (and update docs)