rlr replaces a range of lines in a file
with data from another file or stdin.
The range is delimited by a pair of distinctive lines,
typically specially formatted comment lines in the file to be edited.
For example, the main index for this css package is maintained with this incantation:
mkpodindex -k -l hrefs \
| rlr -f '<!--BEGIN AUTOPODINDEX-->' \
-t '<!--END AUTOPODINDEX-->' \
-i index.html -o index.html -
that places the output of mkpodindex
into the middle of the index.html file.