E[Y|A=1]	= E[Ya=1 | A=1]	by consistency
E[Y|A=0, B=1]	= E[Ya=0 | A=0, B=1]	by consistency
	= E[Ya=0 | A=1, B=1]	by (i)
	= E[Ya=0 | A=1, B=0]	by (ii)
	It follows that E[Y|A=0, B=1] = E[Ya=0 | A=1]