docs: def set_bfield_for_s0(self, s0):
          """Set B to probe a certain harmonic number.
  
          **Call signature**
  
          *s0*
            The harmonic number to probe at the lowest frequency
          Returns
            *self* for convenience in chaining.
  
          This just proceeds from the relation ``nu = s nu_c = s e B / 2 pi m_e
          c``. Since *s* and *nu* scale with each other, if multiple frequencies
          are being probed, the harmonic numbers being probed will scale in the
          same way.
  
          """
          if not (s0 > 0):
              raise ValueError('must have s0 > 0; got %r' % (s0,))
  
          B0 = 2 * np.pi * cgs.me * cgs.c * self.in_vals[IN_VAL_FREQ0] / (cgs.e * s0)
          self.in_vals[IN_VAL_B] = B0
          return self