"And if a man open": This tells me only of opening. Whence do I derive (the same for) digging? It is, therefore, written "if a man dig." But even if this were not stated, I could derive it a fortiori, viz.: If the opener is liable, how much more, so the digger! If you say this you have punished by an a fortiori argument. It is, therefore, written "if he dig" — to teach that punishment is not mandated by an a fortiori argument.