[m-rev.] for post-commit review: avoid a redundant traversal, and improve style

[Zoltan Somogyi]

For review by anyone. The first diff is with -b.

Should we add the maybe_changed type to maybe_succeeded.m?
If yes, what should we rename that module to?

Zoltan.
-------------- next part --------------
A non-text attachment was scrubbed...
Name: Log.common
Type: application/octet-stream
Size: 81 bytes
Desc: not available
URL: <http://lists.mercurylang.org/archives/reviews/attachments/20230404/ffe38f88/attachment-0004.obj>
-------------- next part --------------
A non-text attachment was scrubbed...
Name: DIFF.common
Type: application/octet-stream
Size: 4250 bytes
Desc: not available
URL: <http://lists.mercurylang.org/archives/reviews/attachments/20230404/ffe38f88/attachment-0005.obj>
-------------- next part --------------
A non-text attachment was scrubbed...
Name: Log.ec
Type: application/octet-stream
Size: 407 bytes
Desc: not available
URL: <http://lists.mercurylang.org/archives/reviews/attachments/20230404/ffe38f88/attachment-0006.obj>
-------------- next part --------------
A non-text attachment was scrubbed...
Name: DIFF.ec
Type: application/octet-stream
Size: 23828 bytes
Desc: not available
URL: <http://lists.mercurylang.org/archives/reviews/attachments/20230404/ffe38f88/attachment-0007.obj>