This is what we usually call the disjunction introduction rule (vI):
A
Therefore, A or B.
But this would also be a perfectly legitimate disjunction introduction rule that we could use instead:
A
B
Therefore, A or B.
The latter has the advantage of not pulling things out of thin air. It also makes the disjunction introduction rule much more like the conjunction and implication introduction rules, both of which depend wholly on what has already been given as part of the argument.
