What is the general policy for completing an answer from an other user ? Should I prefer to start an other answer, just saying "As user:xxx said, blabbla, BUT I would like to add blabla..." or should I edit the previous answer, with the possibility to severely annoy its author ? After all, that's her/his answer... so is it well accepted to edit the others' answers ?
For instance, part of the answer should be wrong. Should I delete this part by myself, or make a comment to the author ? (I obviously prefer the second option, but the first one, if it become more accepted, could be time saving I believe).
For instance, an answer could be full of doubtful remarks like "I believe that $x=y$... ", but I know this is for sure. Should I edit the answer such that only "$x=y$" appears ?
I believe there are much more possibilities, I'm asking for your experience and knowledge about the edit of someone else's answer.