YES(?, 2*a + 2*c + 17)

Initial complexity problem:
1:	T:
		(?, 1)    start(a, b, c, d, e, f) -> stop(a, b, c, d, e, f) [ 0 >= a /\ b = c /\ d = a /\ e = f ]
		(?, 1)    start(a, b, c, d, e, f) -> lbl6(a, b, c, d, e, f) [ a >= 1 /\ a >= c /\ b = c /\ d = a /\ e = f ]
		(?, 1)    start(a, b, c, d, e, f) -> lbl121(a, b, c, d, b - d, f) [ a >= 1 /\ c >= a + 1 /\ b = c /\ d = a /\ e = f ]
		(?, 1)    lbl6(a, b, c, d, e, f) -> stop(a, b, c, d, e, f) [ a >= 1 /\ a >= c /\ e = f /\ d = a /\ b = c ]
		(?, 1)    lbl111(a, b, c, d, e, f) -> cut(a, b, c, d, e, f) [ c >= a + 1 /\ a >= 2 /\ e = 0 /\ d = a /\ b = c ]
		(?, 1)    lbl111(a, b, c, d, e, f) -> lbl111(a, b, c, d, e - 1, f) [ e >= 1 /\ a >= e + 1 /\ e >= 0 /\ c >= e + a + 1 /\ a >= e + 2 /\ d = a /\ b = c ]
		(?, 1)    lbl111(a, b, c, d, e, f) -> lbl121(a, b, c, d, e - d, f) [ e >= 1 /\ e >= a /\ e >= 0 /\ c >= e + a + 1 /\ a >= e + 2 /\ d = a /\ b = c ]
		(?, 1)    lbl121(a, b, c, d, e, f) -> cut(a, b, c, d, e, f) [ c >= a + 1 /\ a >= 1 /\ c >= a /\ e = 0 /\ d = a /\ b = c ]
		(?, 1)    lbl121(a, b, c, d, e, f) -> lbl111(a, b, c, d, e - 1, f) [ e >= 1 /\ a >= e + 1 /\ c >= a + 1 /\ a >= 1 /\ e >= 0 /\ c >= e + a /\ d = a /\ b = c ]
		(?, 1)    lbl121(a, b, c, d, e, f) -> lbl121(a, b, c, d, e - d, f) [ e >= 1 /\ e >= a /\ c >= a + 1 /\ a >= 1 /\ e >= 0 /\ c >= e + a /\ d = a /\ b = c ]
		(?, 1)    cut(a, b, c, d, e, f) -> stop(a, b, c, d, e, f) [ a >= 1 /\ c >= a + 1 /\ e = 0 /\ d = a /\ b = c ]
		(1, 1)    start0(a, b, c, d, e, f) -> start(a, c, c, a, f, f)
	start location:	start0
	leaf cost:	0

Testing for unsatisfiable constraints removes the following transition from problem 1:
	lbl111(a, b, c, d, e, f) -> lbl121(a, b, c, d, e - d, f) [ e >= 1 /\ e >= a /\ e >= 0 /\ c >= e + a + 1 /\ a >= e + 2 /\ d = a /\ b = c ]
We thus obtain the following problem:
2:	T:
		(?, 1)    start(a, b, c, d, e, f) -> stop(a, b, c, d, e, f) [ 0 >= a /\ b = c /\ d = a /\ e = f ]
		(?, 1)    start(a, b, c, d, e, f) -> lbl6(a, b, c, d, e, f) [ a >= 1 /\ a >= c /\ b = c /\ d = a /\ e = f ]
		(?, 1)    start(a, b, c, d, e, f) -> lbl121(a, b, c, d, b - d, f) [ a >= 1 /\ c >= a + 1 /\ b = c /\ d = a /\ e = f ]
		(?, 1)    lbl6(a, b, c, d, e, f) -> stop(a, b, c, d, e, f) [ a >= 1 /\ a >= c /\ e = f /\ d = a /\ b = c ]
		(?, 1)    lbl111(a, b, c, d, e, f) -> cut(a, b, c, d, e, f) [ c >= a + 1 /\ a >= 2 /\ e = 0 /\ d = a /\ b = c ]
		(?, 1)    lbl111(a, b, c, d, e, f) -> lbl111(a, b, c, d, e - 1, f) [ e >= 1 /\ a >= e + 1 /\ e >= 0 /\ c >= e + a + 1 /\ a >= e + 2 /\ d = a /\ b = c ]
		(?, 1)    lbl121(a, b, c, d, e, f) -> cut(a, b, c, d, e, f) [ c >= a + 1 /\ a >= 1 /\ c >= a /\ e = 0 /\ d = a /\ b = c ]
		(?, 1)    lbl121(a, b, c, d, e, f) -> lbl111(a, b, c, d, e - 1, f) [ e >= 1 /\ a >= e + 1 /\ c >= a + 1 /\ a >= 1 /\ e >= 0 /\ c >= e + a /\ d = a /\ b = c ]
		(?, 1)    lbl121(a, b, c, d, e, f) -> lbl121(a, b, c, d, e - d, f) [ e >= 1 /\ e >= a /\ c >= a + 1 /\ a >= 1 /\ e >= 0 /\ c >= e + a /\ d = a /\ b = c ]
		(?, 1)    cut(a, b, c, d, e, f) -> stop(a, b, c, d, e, f) [ a >= 1 /\ c >= a + 1 /\ e = 0 /\ d = a /\ b = c ]
		(1, 1)    start0(a, b, c, d, e, f) -> start(a, c, c, a, f, f)
	start location:	start0
	leaf cost:	0

Repeatedly removing leaves of the complexity graph in problem 2 produces the following problem:
3:	T:
		(?, 1)    start(a, b, c, d, e, f) -> lbl121(a, b, c, d, b - d, f) [ a >= 1 /\ c >= a + 1 /\ b = c /\ d = a /\ e = f ]
		(?, 1)    lbl111(a, b, c, d, e, f) -> lbl111(a, b, c, d, e - 1, f) [ e >= 1 /\ a >= e + 1 /\ e >= 0 /\ c >= e + a + 1 /\ a >= e + 2 /\ d = a /\ b = c ]
		(?, 1)    lbl121(a, b, c, d, e, f) -> lbl111(a, b, c, d, e - 1, f) [ e >= 1 /\ a >= e + 1 /\ c >= a + 1 /\ a >= 1 /\ e >= 0 /\ c >= e + a /\ d = a /\ b = c ]
		(?, 1)    lbl121(a, b, c, d, e, f) -> lbl121(a, b, c, d, e - d, f) [ e >= 1 /\ e >= a /\ c >= a + 1 /\ a >= 1 /\ e >= 0 /\ c >= e + a /\ d = a /\ b = c ]
		(1, 1)    start0(a, b, c, d, e, f) -> start(a, c, c, a, f, f)
	start location:	start0
	leaf cost:	6

Repeatedly propagating knowledge in problem 3 produces the following problem:
4:	T:
		(1, 1)    start(a, b, c, d, e, f) -> lbl121(a, b, c, d, b - d, f) [ a >= 1 /\ c >= a + 1 /\ b = c /\ d = a /\ e = f ]
		(?, 1)    lbl111(a, b, c, d, e, f) -> lbl111(a, b, c, d, e - 1, f) [ e >= 1 /\ a >= e + 1 /\ e >= 0 /\ c >= e + a + 1 /\ a >= e + 2 /\ d = a /\ b = c ]
		(?, 1)    lbl121(a, b, c, d, e, f) -> lbl111(a, b, c, d, e - 1, f) [ e >= 1 /\ a >= e + 1 /\ c >= a + 1 /\ a >= 1 /\ e >= 0 /\ c >= e + a /\ d = a /\ b = c ]
		(?, 1)    lbl121(a, b, c, d, e, f) -> lbl121(a, b, c, d, e - d, f) [ e >= 1 /\ e >= a /\ c >= a + 1 /\ a >= 1 /\ e >= 0 /\ c >= e + a /\ d = a /\ b = c ]
		(1, 1)    start0(a, b, c, d, e, f) -> start(a, c, c, a, f, f)
	start location:	start0
	leaf cost:	6

A polynomial rank function with
	Pol(start) = 7
	Pol(lbl121) = 7
	Pol(lbl111) = 6
	Pol(start0) = 7
orients all transitions weakly and the transition
	lbl121(a, b, c, d, e, f) -> lbl111(a, b, c, d, e - 1, f) [ e >= 1 /\ a >= e + 1 /\ c >= a + 1 /\ a >= 1 /\ e >= 0 /\ c >= e + a /\ d = a /\ b = c ]
strictly and produces the following problem:
5:	T:
		(1, 1)    start(a, b, c, d, e, f) -> lbl121(a, b, c, d, b - d, f) [ a >= 1 /\ c >= a + 1 /\ b = c /\ d = a /\ e = f ]
		(?, 1)    lbl111(a, b, c, d, e, f) -> lbl111(a, b, c, d, e - 1, f) [ e >= 1 /\ a >= e + 1 /\ e >= 0 /\ c >= e + a + 1 /\ a >= e + 2 /\ d = a /\ b = c ]
		(7, 1)    lbl121(a, b, c, d, e, f) -> lbl111(a, b, c, d, e - 1, f) [ e >= 1 /\ a >= e + 1 /\ c >= a + 1 /\ a >= 1 /\ e >= 0 /\ c >= e + a /\ d = a /\ b = c ]
		(?, 1)    lbl121(a, b, c, d, e, f) -> lbl121(a, b, c, d, e - d, f) [ e >= 1 /\ e >= a /\ c >= a + 1 /\ a >= 1 /\ e >= 0 /\ c >= e + a /\ d = a /\ b = c ]
		(1, 1)    start0(a, b, c, d, e, f) -> start(a, c, c, a, f, f)
	start location:	start0
	leaf cost:	6

A polynomial rank function with
	Pol(start) = V_2 - V_4 + 1
	Pol(lbl121) = V_5 + 1
	Pol(lbl111) = V_5 + 2
	Pol(start0) = -V_1 + V_3 + 1
orients all transitions weakly and the transitions
	lbl121(a, b, c, d, e, f) -> lbl121(a, b, c, d, e - d, f) [ e >= 1 /\ e >= a /\ c >= a + 1 /\ a >= 1 /\ e >= 0 /\ c >= e + a /\ d = a /\ b = c ]
	lbl111(a, b, c, d, e, f) -> lbl111(a, b, c, d, e - 1, f) [ e >= 1 /\ a >= e + 1 /\ e >= 0 /\ c >= e + a + 1 /\ a >= e + 2 /\ d = a /\ b = c ]
strictly and produces the following problem:
6:	T:
		(1, 1)            start(a, b, c, d, e, f) -> lbl121(a, b, c, d, b - d, f) [ a >= 1 /\ c >= a + 1 /\ b = c /\ d = a /\ e = f ]
		(a + c + 1, 1)    lbl111(a, b, c, d, e, f) -> lbl111(a, b, c, d, e - 1, f) [ e >= 1 /\ a >= e + 1 /\ e >= 0 /\ c >= e + a + 1 /\ a >= e + 2 /\ d = a /\ b = c ]
		(7, 1)            lbl121(a, b, c, d, e, f) -> lbl111(a, b, c, d, e - 1, f) [ e >= 1 /\ a >= e + 1 /\ c >= a + 1 /\ a >= 1 /\ e >= 0 /\ c >= e + a /\ d = a /\ b = c ]
		(a + c + 1, 1)    lbl121(a, b, c, d, e, f) -> lbl121(a, b, c, d, e - d, f) [ e >= 1 /\ e >= a /\ c >= a + 1 /\ a >= 1 /\ e >= 0 /\ c >= e + a /\ d = a /\ b = c ]
		(1, 1)            start0(a, b, c, d, e, f) -> start(a, c, c, a, f, f)
	start location:	start0
	leaf cost:	6

Complexity upper bound 2*a + 2*c + 17

Time: 0.368 sec (SMT: 0.348 sec)