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).
: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?