YES(?, a + b + 3)

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

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

A polynomial rank function with
	Pol(sumto) = -V_1 + V_2 + 1
	Pol(start) = -V_1 + V_2 + 1
orients all transitions weakly and the transition
	sumto(a, b) -> sumto(a + 1, b) [ b >= a ]
strictly and produces the following problem:
3:	T:
		(a + b + 1, 1)    sumto(a, b) -> sumto(a + 1, b) [ b >= a ]
		(1, 1)            start(a, b) -> sumto(a, b)
	start location:	start
	leaf cost:	1

Complexity upper bound a + b + 3

Time: 0.055 sec (SMT: 0.052 sec)