Which policy to complete an answer made by someone else ?

Question

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.

Answer 1

This has been asked multiple times on the main meta. In this case I'll just copy my answer over there.

• If its just a bit of more info (read:a paragraph), add it and leave a comment.
• If it's a completely new bundle of stuff, but is based on stuff gleaned from the answer, write an addendum answer as described above. Hat it with "This is in addition to @JonSkeet's answer". You may also comment on the original answer and ask the poster to incorporate your answer if s/he wishes--and then delete your answer once the deal is done (Something smilar could have happened here, though it didn't).

Examples:

Factors affecting torque and RPM of a motor

How can one imagine curled up dimensions?

Penetration versus Frequency

Will an emf be induced across the ends of a stationary metal rod placed in a time-varying magnetic field?

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In the case of wrong answers (or parts of answers), leave a comment. Try not to make radical changes in edits -- you can tell the OP in a comment to make them himself.