YES(?, a + 2)

Initial complexity problem:
1:	T:
		(?, 1)    f2(a, b) -> f3(a, 1) [ 0 >= a ]
		(1, 1)    f0(a, b) -> f2(a, 0)
		(?, 1)    f2(a, b) -> f2(a - 1, b) [ a >= 1 ]
	start location:	f0
	leaf cost:	0

Repeatedly removing leaves of the complexity graph in problem 1 produces the following problem:
2:	T:
		(1, 1)    f0(a, b) -> f2(a, 0)
		(?, 1)    f2(a, b) -> f2(a - 1, b) [ a >= 1 ]
	start location:	f0
	leaf cost:	1

A polynomial rank function with
	Pol(f0) = V_1
	Pol(f2) = V_1
orients all transitions weakly and the transition
	f2(a, b) -> f2(a - 1, b) [ a >= 1 ]
strictly and produces the following problem:
3:	T:
		(1, 1)    f0(a, b) -> f2(a, 0)
		(a, 1)    f2(a, b) -> f2(a - 1, b) [ a >= 1 ]
	start location:	f0
	leaf cost:	1

Complexity upper bound a + 2

Time: 0.087 sec (SMT: 0.084 sec)