YES(?, b + 2)

Initial complexity problem:
1:	T:
		(1, 1)    l0(a, b) -> l1(0, b)
		(?, 1)    l1(a, b) -> l1(a + 1, b - 1) [ b >= 1 ]
		(?, 1)    l1(a, b) -> l2(a, b) [ 0 >= b ]
	start location:	l0
	leaf cost:	0

Repeatedly removing leaves of the complexity graph in problem 1 produces the following problem:
2:	T:
		(1, 1)    l0(a, b) -> l1(0, b)
		(?, 1)    l1(a, b) -> l1(a + 1, b - 1) [ b >= 1 ]
	start location:	l0
	leaf cost:	1

A polynomial rank function with
	Pol(l0) = V_2
	Pol(l1) = V_2
orients all transitions weakly and the transition
	l1(a, b) -> l1(a + 1, b - 1) [ b >= 1 ]
strictly and produces the following problem:
3:	T:
		(1, 1)    l0(a, b) -> l1(0, b)
		(b, 1)    l1(a, b) -> l1(a + 1, b - 1) [ b >= 1 ]
	start location:	l0
	leaf cost:	1

Complexity upper bound b + 2

Time: 0.084 sec (SMT: 0.081 sec)