I don't have much experience in creating environments, and my understanding of them allows me to create environments which do something at the beginning and at the end (like for instance, in a "proof" environment, I can begin it with "Proof." and end it with a little square in the bottom right.
What I want to do now is slighty more complicated (so it seems to me), I just want something looking like
Proof. | | | content here | | | end of proof
I want those vertical dashes to be a straight line from bottom to top. Any way I could be able to do that?
mdframed, since it allows even for page breaking and setting particular borders. In that regard, see Double vertical bars alongside statements of theorems.
content herepushed to the right. That's exactly what the link does (see the image in the linked answer when you add some text before/after the
theoenvironment). You don't want that?