Notes on ZipDiff
================
NB* May need method of diff-ing a subtree
eg if ZipFile contains Docs.Comp.Acorn.* and need to compare with
Docs.Comp.Acorn.*, not to all of Docs.

Assumes: ZipFile is ordered in scan order, ie:
Docs.file1                    Docs.file1
Docs.file2                    Docs.file2
Docs.subdir.file3    and not: Docs.textfile
Docs.subdir.file4             Docs.subdir.file3
Docs.textfile                 Docs.subdir.file4
...tho, it may work with both styles, as long as they are ordered.

Methodology:
* Select directory
* Find first entry for that directory in zipfile
  (or first entry after where it should be if missing)
* Scan directory structure
  * For each entry:
  * Look for matching entry in zipfile, starting at ptr->"dirhead"  
  * If missing from zipfile -> add to addfile (file or dir)
  * If present in zipfile
    * If different types -> add to delfile and addfile
    * If same types
      * compare mod dates  (currently, -t option may not be needed)
      * if entry newer than zipfile entry -> add to addfile
  * If dir, *dir, doscan, *dir ^
* Directory has been scanned.
* Scan zipfile directory from ptr->"dirhead"
  * For each entry:
  * Look for matching entry in filing system
  * If missing from fileing system -> add to delfile
*



Notes for later:
Currently, if zipping by stdin or incfile:
  if entered name=dir, does zip *Dir to it and Scan, etc?
Should it do this?
Should it not just enter info about Dir into zipfile?
v1.20 appears to add dirinfo, not descend into it.
Descending only happens when travaersing automatically, from *zip dirname


Notes:
delfile must be run before adding new entries at "other end"


Future enhancements:
option to add "different date", not just "recent"
option to add if access changed (cf next:)
option to add if dates identical, but load/exec/etc different
output a delfile
don't use *Exec, so can be part of a Execed batch
don't use *Spool, so batch output can be captured