r-numbered channels take precedence over the ones
     on lower-numbered channels. This effect has a large number of
     different implementations, but the one described here has the
     widest usage.

N.B. This document should be fairly accurate now, but as the module
format is more of an observation than a standard, a couple of effects
cannot be relied upon to act exactly the same from tracker to tracker
(especially if the tracker is not for the Amiga). It is probably better
to use this document as a gu