bug#25870: give an example to show what lines join -a and -v are talking about

[積丹尼 Dan Jacobson]

In (info "(coreutils) join invocation")

‘-a FILE-NUMBER’
     Print a line for each unpairable line in file FILE-NUMBER (either
     ‘1’ or ‘2’), in addition to the normal output.

OK but say if we can use both: -a 1 -a 2? (Answer: yes)
And if so how to tell which lines are from which file (Answer: by their content 
only).

‘-v FILE-NUMBER’
     Print a line for each unpairable line in file FILE-NUMBER (either
     ‘1’ or ‘2’), instead of the normal output.

OK but say if we can use both: -v 1 -v 2?
And if so how to tell which lines are from which file.

And in fact give an example to show what lines -a and -v are talking about!