X =	00	01	02	03	04	05	06	07	08	09	10	11	12	13	14	15
Y =	00	01	02	03	04	05	06	07	08	09	10	11	12	13	14	15
X ↔ Y

Ext-GCD(0, 4) → Twos = 2 → Ext-GCD(0, 1) :

i	GCD			Bezout				AOdd :						AEven :
	A	↔	B	ax	ay	bx	by	|A-B|	A↔B	Sx	Sy	ax ← Sx'	ay ← Sy'	A ← A/2	ax ← ax + Y	ay ← ay + X	ax ← ax/2	ay ← ay/2
0	0		1	1	0	0	1
1	0		1	1	0	0	1	1
2	0		1	1	0	0	1	1
3	0		1	1	0	0	1	1
4	0		1	1	0	0	1	1
5	0		1	1	0	0	1	1
6	0		1	1	0	0	1	1
7	1	↔	0	0	1	1	0	1
GCD = 1; P, Q Prohibited (X = 0, so Q is negative)