YES(?, 2*a + 3)

Initial complexity problem:
1:	T:
		(?, 1)    f0(a, b, c) -> f3(-a, b, c)
		(?, 1)    f3(a, b, c) -> f7(0, d, c) [ a = 0 ]
		(?, 1)    f4(a, b, c) -> f7(0, d, c) [ a = 0 ]
		(1, 1)    f6(a, b, c) -> f4(a, b, 1) [ a >= 1 ]
		(?, 1)    f3(a, b, c) -> f4(-a - 1, b, 1) [ 0 >= a + 1 /\ 0 >= c ]
		(?, 1)    f3(a, b, c) -> f4(-a - 1, b, 1) [ 0 >= a + 1 /\ c >= 2 ]
		(?, 1)    f3(a, b, c) -> f4(-a - 1, b, 1) [ a >= 1 /\ 0 >= c ]
		(?, 1)    f3(a, b, c) -> f4(-a - 1, b, 1) [ a >= 1 /\ c >= 2 ]
		(?, 1)    f4(a, b, c) -> f3(-a + 1, b, 0) [ 0 >= a + 1 /\ c = 1 ]
		(?, 1)    f4(a, b, c) -> f3(-a + 1, b, 0) [ a >= 1 /\ c = 1 ]
		(?, 1)    f5(a, b, c) -> f3(-a + 1, b, 0) [ 0 >= a + 1 /\ c = 1 ]
		(?, 1)    f5(a, b, c) -> f3(-a + 1, b, 0) [ a >= 1 /\ c = 1 ]
	start location:	f6
	leaf cost:	0

Repeatedly removing leaves of the complexity graph in problem 1 produces the following problem:
2:	T:
		(?, 1)    f0(a, b, c) -> f3(-a, b, c)
		(1, 1)    f6(a, b, c) -> f4(a, b, 1) [ a >= 1 ]
		(?, 1)    f3(a, b, c) -> f4(-a - 1, b, 1) [ 0 >= a + 1 /\ 0 >= c ]
		(?, 1)    f3(a, b, c) -> f4(-a - 1, b, 1) [ 0 >= a + 1 /\ c >= 2 ]
		(?, 1)    f3(a, b, c) -> f4(-a - 1, b, 1) [ a >= 1 /\ 0 >= c ]
		(?, 1)    f3(a, b, c) -> f4(-a - 1, b, 1) [ a >= 1 /\ c >= 2 ]
		(?, 1)    f4(a, b, c) -> f3(-a + 1, b, 0) [ 0 >= a + 1 /\ c = 1 ]
		(?, 1)    f4(a, b, c) -> f3(-a + 1, b, 0) [ a >= 1 /\ c = 1 ]
		(?, 1)    f5(a, b, c) -> f3(-a + 1, b, 0) [ 0 >= a + 1 /\ c = 1 ]
		(?, 1)    f5(a, b, c) -> f3(-a + 1, b, 0) [ a >= 1 /\ c = 1 ]
	start location:	f6
	leaf cost:	2

Testing for reachability in the complexity graph removes the following transitions from problem 2:
	f0(a, b, c) -> f3(-a, b, c)
	f3(a, b, c) -> f4(-a - 1, b, 1) [ 0 >= a + 1 /\ c >= 2 ]
	f3(a, b, c) -> f4(-a - 1, b, 1) [ a >= 1 /\ 0 >= c ]
	f3(a, b, c) -> f4(-a - 1, b, 1) [ a >= 1 /\ c >= 2 ]
	f4(a, b, c) -> f3(-a + 1, b, 0) [ 0 >= a + 1 /\ c = 1 ]
	f5(a, b, c) -> f3(-a + 1, b, 0) [ 0 >= a + 1 /\ c = 1 ]
	f5(a, b, c) -> f3(-a + 1, b, 0) [ a >= 1 /\ c = 1 ]
We thus obtain the following problem:
3:	T:
		(?, 1)    f3(a, b, c) -> f4(-a - 1, b, 1) [ 0 >= a + 1 /\ 0 >= c ]
		(?, 1)    f4(a, b, c) -> f3(-a + 1, b, 0) [ a >= 1 /\ c = 1 ]
		(1, 1)    f6(a, b, c) -> f4(a, b, 1) [ a >= 1 ]
	start location:	f6
	leaf cost:	2

A polynomial rank function with
	Pol(f3) = -V_1
	Pol(f4) = V_1
	Pol(f6) = V_1
orients all transitions weakly and the transitions
	f4(a, b, c) -> f3(-a + 1, b, 0) [ a >= 1 /\ c = 1 ]
	f3(a, b, c) -> f4(-a - 1, b, 1) [ 0 >= a + 1 /\ 0 >= c ]
strictly and produces the following problem:
4:	T:
		(a, 1)    f3(a, b, c) -> f4(-a - 1, b, 1) [ 0 >= a + 1 /\ 0 >= c ]
		(a, 1)    f4(a, b, c) -> f3(-a + 1, b, 0) [ a >= 1 /\ c = 1 ]
		(1, 1)    f6(a, b, c) -> f4(a, b, 1) [ a >= 1 ]
	start location:	f6
	leaf cost:	2

Complexity upper bound 2*a + 3

Time: 0.267 sec (SMT: 0.255 sec)