If no subject entered in post, default to "Re: Last subject entered"

[007]

I have my forum set up to automatically use

"Re: title of thread"

for the subject of each post entered after the original post. i am trying to make it so it defaults to

"Re: last subject entered"

So if the thread was titled

"Thread Title"

then the reply subject would default to

"Re: Thread Title"

but I want to make it so if someone puts in their own subject, then it would default to

"Re: Last subject"

Does that make sense? I also am wondering if it possible to show the subject of posts on the profile under last post, instead of the thread title where the post was made... Any suggestions?