If a player steps on the sideline or endline and is on the field before the snap there is no foul.

I think I know --but that's why I asked this question.
According to the case book - pg - 74 --

Case 10 - Prior to a change of possession or when there is no change , a player of A or K who goes out of bounds and then comes back in during the down commits illegal participation. This is a 15-yard penalty -enforced under the all but one rule.

So - my only question now is -- what if he steps on the line before the snap - and back in the field of play --
we do not kill it ? we simply let the play go on - right?
I think years ago there was a case book rule on this - and I think we did kill it as ill. substitution. May not be the case now.