I would like to be able to format my proof "like a horizontal tree". And because pictures are better than words, here is what I would like to do :
Proof : To prove the big theorem, we first need to prove that XXX: | To prove XXX, let's begin to prove | YYY: | | YYY is trivial when you consider the | | lemma 42, which is usable because of | | the hypothesis 1.0. So YYY is true! | Now, we would like also the property ZZZ | Let's proove it ! | | To prove ZZZ, we can build a path and "paf !" | | ZZZ becomes trivial ! | Let's now notice that applying YYY inside the | | property YYY will let us conclude on the | | validity of XXX. □
So the idea is to indent the proof within several levels to always be clear about what we are proving, and show the indentation with a vertical border on the left. The result should looks like what you got when you enter in a While/If block within the algorithm2e package (but of course I don't want the If/While words):
Thank you for your help !
See this more modern answer for a semi-working solution Robustly add a border to the left of a text spanning several pages