Substitution
With fourth and 5 from B's 40, B12 enters and takes a position as a punt-returner
and the replaced player leaves the field. B12 then realizes A is not going to
punt and he returns to his team box.
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RULING: Illegal substitution. B12 became a player when he positioned in the
formation and he must remain in the game for at least one down unless a penalty
is accepted, a dead-ball foul occurs, there is a charged time-out or the period
ends. (3.7.3C)Casebook