Find a global ordering for layers in a HoloMap of CompositeOverlay
   types.

def layer_sort(hmap):
   """
   Find a global ordering for layers in a HoloMap of CompositeOverlay
   types.
   """
   orderings = {}
   for o in hmap:
      okeys = [get_overlay_spec(o, k, v) for k, v in o.data.items()]
      if len(okeys) == 1 and not okeys[0] in orderings:
         orderings[okeys[0]] = []
      else:
         orderings.update({k: [] if k == v else [v] for k, v in zip(okeys[1:], okeys)})
   return [i for g in sort_topologically(orderings) for i in sorted(g)]