code: def chi_p_from_xi1_xi2(xi1, xi2):
      
      xi1, xi2, input_is_array = ensurearray(xi1, xi2)
      chi_p = copy.copy(xi1)
      mask = xi1 < xi2
      chi_p[mask] = xi2[mask]
      return formatreturn(chi_p, input_is_array)
docs: Returns effective precession spin from xi1 and xi2.