code: def _time_variable_part(epoch, ref_epoch, trnd, periodic):
      
      delta_t = epoch - ref_epoch
      trend = trnd * delta_t
      periodic_sum = _np.zeros_like(trnd)
      for period in periodic:
          for trifunc in periodic[period]:
              coeffs = periodic[period][trifunc]
              if trifunc == 'acos':
                  periodic_sum += coeffs * _np.cos(2 * _np.pi / period * delta_t)
              elif trifunc == 'asin':
                  periodic_sum += coeffs * _np.sin(2 * _np.pi / period * delta_t)
      return trend + periodic_sum
docs: Return sum of the time-variable part of the coefficients
  
      The formula is:
      G(t) = G(t0) + trnd*(t-t0) +
          asin1*sin(2pi/p1 * (t-t0)) + acos1*cos(2pi/p1 * (t-t0)) +
          asin2*sin(2pi/p2 * (t-t0)) + acos2*cos(2pi/p2 * (t-t0))
  
      This function computes all terms after G(t0).