IMPORTANT NOTE ON VELOCITY: 

One should not use very large Doppler velocities (thousands of
kilometers) to achieve red-shift corrections of frequencies. Instead
one should enter the red-shifted frequencies in the line catalog (or
with the RECEIVER command) and use Doppler velocity of 0.0.

The alternative approach (with very large Doppler velocities) needs
special methods and attention in the data processing, which are "*not*
implemented in CLASS because it is better to observe in such a way
that minimum modification to the data is done later on" (J. Pety).
For a full discussion of this question see: Gordon et al. 1992, A&A,
264, 337 in Sect. 6

