Let's say I have a 1/1 with first strike that's attacking and blocked by a vanilla 4/4.
It deals first strike damage, but that's not enough to kill the defending creature. Right now, the attacking creature will have to take damage from the blocking creature.
Before the attacking creature takes that damage, can I “finish off” the blocking creature by casting Lightning Bolt and avoid it taking damage?
I am unsure whether,and when,a player gains priority during that step.
What, in my opinion, distinguishes this question from the others with which it can be associated, is the exact moment in which the players have the priority to cast spells again, which I believe should be placed exactly between the conclusion of the "first strike damage assignment phase" - in which first strike creatures assign their damage - and the "ordinary combat phase" - in which other creatures without first strike assign their damage.
It should be specified when players gain priority and it is possible to cast spells able to damage creatures during combat, and if it is possible to do so; and, above all, if these damages due to spells are assigned without the possibility,for creatures without first strike, to respond to them.
I don't think these are trivial things ...