map-extents, etc. What is the order of extents in these functions?

[Stephen J. Turnbull]

Alan Mackenzie writes:

 > It would be nice to use map-extents for the forward version here.  Can I
 > assume that map-extents processes extents in "display order"?  The
 > doc string and the fine manual forget to mention this.

Yes, you may assume that.

 > For the backward version, is there a way I could persuade a mapping
 > function to process the extents in inverse order ("e-display order", I
 > think it's called)?

Not as far as I can tell.  The underlying mapper map_extents()
includes an assert "from <= to".

Note that e-order is not (reverse (extent-list)); it's the dual of
display order in the sense that if the ends (not starts) differ, the
one with the greater end comes earlier in the order.  If the ends are
equal, the longer extent comes earlier.  Are you sure you want
e-order?

 > More generally, do functions like extent-list always return extents in
 > display order?

Yes, they do.