MAYBE

Initial complexity problem:
1:	T:
		(?, 1)    f2(a, b) -> f2(d - 1, b) [ 0 >= c /\ 0 >= a + 1 ]
		(?, 1)    f2(a, b) -> f2(d + 1, b) [ c >= 2 /\ 0 >= a + 1 ]
		(?, 1)    f2(a, b) -> f2(d - 1, b) [ 0 >= c + 2 /\ a >= 1 ]
		(?, 1)    f2(a, b) -> f2(d + 1, b) [ c >= 0 /\ a >= 1 ]
		(1, 1)    f3(a, b) -> f2(a, b)
		(?, 1)    f2(a, b) -> f300(0, d) [ 0 >= a + 1 ]
		(?, 1)    f2(a, b) -> f300(0, d) [ a >= 1 ]
		(?, 1)    f2(a, b) -> f300(a, d) [ a = 0 ]
	start location:	f3
	leaf cost:	0

Repeatedly removing leaves of the complexity graph in problem 1 produces the following problem:
2:	T:
		(?, 1)    f2(a, b) -> f2(d - 1, b) [ 0 >= c /\ 0 >= a + 1 ]
		(?, 1)    f2(a, b) -> f2(d + 1, b) [ c >= 2 /\ 0 >= a + 1 ]
		(?, 1)    f2(a, b) -> f2(d - 1, b) [ 0 >= c + 2 /\ a >= 1 ]
		(?, 1)    f2(a, b) -> f2(d + 1, b) [ c >= 0 /\ a >= 1 ]
		(1, 1)    f3(a, b) -> f2(a, b)
	start location:	f3
	leaf cost:	3

Complexity upper bound ?

Time: 0.223 sec (SMT: 0.209 sec)