AB -> CD	AB -> X1B
		X1B -> X1X2
		X1X2 -> CX2
		CX2 -> CD

ABC -> DEF	ABC -> X1BC
		X1BC -> X1X2C
		X1X2C -> X1X2X3
		X1X2X3 -> DX2X3
		DX2X3 -> DEX3
		DEX3 -> DEF