Filling paragraph with character leading each line

[Haines Brown]

I gather that in C mode, when a comment paragraph is filled, a leading \
is automatically inserted at the beginning of each line. I need to do
something similar in text mode.

For example in a TeX document I might want to comment a paragraph (other
than by puting it in a begin{comment} environment) by starting each line
with a % character. So how can I refill the paragraph that has its first
few lines starting with % so that the % will appear at the start of all
its lines? At the present a refill keeps a % at the start of each line
where it already exists, but fails to extend the rule to the remaining
lines in the paragraph that lack a leading %.

Haines Brown