Document the seen bit.
seen -> refcount. All regression tests pass.
Explain the 'tail' bit.
Remove obsolete caveat.
Reinstate path sorting before directory packing.

This time, enabling ORDERDEBUG shows we're not comparing wild pointers.
Cosmetic rearrangement.  All regression tests pass.
Eliminate an unused static counter.

All regression tests pass under both shell and within Emacs.
Use what we now know about refcount for space compression.

All regression tests pass.
Cosmetic fix.  All regression tests pass.
News update.
Document internals, and one cosmetic change.  All regression tests pass.
Remove unused 'tagged' but.  Goes with the big 'Remove unused code.' change.

All regression tests pass.
More working-set reduction.  All regression tests pass.