YES(?, a + 4)

Initial complexity problem:
1:	T:
		(?, 1)    start(a, b, c, d, e, f) -> stop(a, b, c, d, e, f) [ 0 >= a /\ b = a /\ c = d /\ e = f ]
		(?, 1)    start(a, b, c, d, e, f) -> lbl71(a, b - 1, c - 1, d, e + 1, f) [ a >= 1 /\ b = a /\ c = d /\ e = f ]
		(?, 1)    lbl71(a, b, c, d, e, f) -> stop(a, b, c, d, e, f) [ d >= c + 1 /\ b = 0 /\ e + c = f + d /\ a + c = d ]
		(?, 1)    lbl71(a, b, c, d, e, f) -> lbl71(a, b - 1, c - 1, d, e + 1, f) [ a + c >= d + 1 /\ d >= c + 1 /\ a + c >= d /\ e + c = d + f /\ b + d = a + c ]
		(1, 1)    start0(a, b, c, d, e, f) -> start(a, a, d, d, f, f)
	start location:	start0
	leaf cost:	0

Repeatedly removing leaves of the complexity graph in problem 1 produces the following problem:
2:	T:
		(?, 1)    start(a, b, c, d, e, f) -> lbl71(a, b - 1, c - 1, d, e + 1, f) [ a >= 1 /\ b = a /\ c = d /\ e = f ]
		(?, 1)    lbl71(a, b, c, d, e, f) -> lbl71(a, b - 1, c - 1, d, e + 1, f) [ a + c >= d + 1 /\ d >= c + 1 /\ a + c >= d /\ e + c = d + f /\ b + d = a + c ]
		(1, 1)    start0(a, b, c, d, e, f) -> start(a, a, d, d, f, f)
	start location:	start0
	leaf cost:	2

Repeatedly propagating knowledge in problem 2 produces the following problem:
3:	T:
		(1, 1)    start(a, b, c, d, e, f) -> lbl71(a, b - 1, c - 1, d, e + 1, f) [ a >= 1 /\ b = a /\ c = d /\ e = f ]
		(?, 1)    lbl71(a, b, c, d, e, f) -> lbl71(a, b - 1, c - 1, d, e + 1, f) [ a + c >= d + 1 /\ d >= c + 1 /\ a + c >= d /\ e + c = d + f /\ b + d = a + c ]
		(1, 1)    start0(a, b, c, d, e, f) -> start(a, a, d, d, f, f)
	start location:	start0
	leaf cost:	2

A polynomial rank function with
	Pol(start) = V_1 + V_3 - V_4
	Pol(lbl71) = V_1 + V_3 - V_4 + 1
	Pol(start0) = V_1
orients all transitions weakly and the transition
	lbl71(a, b, c, d, e, f) -> lbl71(a, b - 1, c - 1, d, e + 1, f) [ a + c >= d + 1 /\ d >= c + 1 /\ a + c >= d /\ e + c = d + f /\ b + d = a + c ]
strictly and produces the following problem:
4:	T:
		(1, 1)    start(a, b, c, d, e, f) -> lbl71(a, b - 1, c - 1, d, e + 1, f) [ a >= 1 /\ b = a /\ c = d /\ e = f ]
		(a, 1)    lbl71(a, b, c, d, e, f) -> lbl71(a, b - 1, c - 1, d, e + 1, f) [ a + c >= d + 1 /\ d >= c + 1 /\ a + c >= d /\ e + c = d + f /\ b + d = a + c ]
		(1, 1)    start0(a, b, c, d, e, f) -> start(a, a, d, d, f, f)
	start location:	start0
	leaf cost:	2

Complexity upper bound a + 4

Time: 0.280 sec (SMT: 0.268 sec)