(Exodus, Ibid. 22) "And if an altar of stones you make for Me": R. Yishmael says: Every "if" in the Torah connotes optionality, except for three; (Leviticus 2:14) "And if you offer an offering of first-fruits" — This is mandatory. You say it is mandatory, but perhaps it is optional. (This is not so, for) it is written (Ibid.) "You shall offer the offering of your first-fruits." It is mandatory, not optional.