Trouble with post selector

I am starting off with

p { a { color: red; }

a { color: blue; &:hover { opacity: 0.5; } } }

The task is to make the <a> tag within the .footer class purple, which I am assuming meas that I need to insert

.footer & { color: purple; }

If that is correct, then it makes sense to me to put it after color: red; in the <a> tag selector at the top, like so.

p { a { color: red; .footer & { color: purple; } }

a { color: blue; &:hover { opacity: 0.5; } } }

That way, the "&" will select the <p> and <a> tags above it. When written out, it seems to me like it should come out to .footer p a {}, which would be the proper way to select the <a> tag in the footer for styling in regular CSS. However, I've tried not only that, but many other configurations that don't make nearly as much sense, and I can only conclude that I am either selecting the <a> tag in the .footer class incorrectly (using the post selector incorrectly), or there is somewhere it can be placed within this code that I somehow haven't though about.

The answer is probably quite simple, but any help would be appreciated nonetheless.