docs: def mu_e(X):
      ''' 
      mean molecular weight per free electron, assuming full ionisation, and
      approximating mu_i/Z_i ~ 2 for all elements heavier then Helium.
      
      (Kippenhahn & Weigert, Ch 13.1, Eq. 13.8)
      
      Parameters
      ----------
      X : float
          Mass fraction of H.
      
      '''
  
      try:
          mu_e = old_div(2.,(1.+X))
      except TypeError:
          X=np.array([X])
          mu_e = old_div(2.,(1.+X))
  
      return mu_e