  Fix a Calabi-Yau 3-fold $X$ satisfying the Bogomolov-Gieseker conjecture of
Bayer-Macr\`i-Toda, such as the quintic 3-fold. We express Joyce's generalised
DT invariants counting Gieseker semistable sheaves of any rank $r\ge1$ on $X$
in terms of those counting sheaves of rank 0 and pure dimension 2.
  The basic technique is to reduce the ranks of sheaves by replacing them by
the cokernels of their Mochizuki/Joyce-Song pairs and then use wall crossing to
handle their stability.
