>Maybe better would be for undo "U" to only undo and skip non-replacement
>entries in the stack?  (Implementation-wise this means looping until
>finding a previous non-replacement entry in the stack).
I agree, its better undo 'just' undo. Fixed in the new patch
(replace-3.patch).

>A minor comment is that to save cons cells you could just put both new
>elements shallow as elements 3 and 4 in the stack instead of adding
>a list of them.
Sure, thank you for remind me this. Applied.

>Another minor suggestions is for better names you could rename
>the prefix -solved-regexp to -replaced
Done. Thank you.

>Also we need this feature to be documented in etc/NEWS.
>Have you already signed FSF copyright assignment papers?
I didn't. I would like to sign. Maybe someone could guide me how to
complete such process.