I've noticed that when I have several files open in Vim, and one of the files has been changed by an external program, :wa doesn't actually try to write into that file. OTOH, going to that file and explicitly saying :w does try to actually write (I get asked whether I want to overwrite the changes).
I'd like :wa to behave like "for all files, do :w", so that I got asked whether I want to overwrite all the externally-changed files.
How can I achieve this?