old_contents: from __future__ import division, print_function
  
  from math import pi
  
  mu_0 = 4*pi*10**-7
  
new_contents: from __future__ import division, print_function
  
  from math import pi
  
  c = 299792458
  
  mu_0 = 4*pi*10**-7
  eps_0 = 1/(mu_0*c**2)
  
subject: add more constants