code: def add_line(psr,f,A,offset=0.5):
      
      
      t = psr.toas()
      t0 = offset * (N.max(t) - N.min(t))
      sine = A * N.cos(2 * math.pi * f * day * (t - t0))
  
      psr.stoas[:] += sine / day
docs: Add a line of frequency `f` [Hz] and amplitude `A` [s],
      with origin at a fraction `offset` through the dataset.