MAYBE

Initial complexity problem:
1:	T:
		(?, 1)    f2(a, b) -> f1(c, d) [ 0 >= c + 1 /\ a = 14 ]
		(?, 1)    f2(a, b) -> f2(c, b) [ c >= 0 /\ a = 14 ]
		(?, 1)    f2(a, b) -> f1(a - 1, c) [ a >= 15 /\ 0 >= a ]
		(?, 1)    f2(a, b) -> f1(a - 1, c) [ 13 >= a /\ 0 >= a ]
		(?, 1)    f2(a, b) -> f2(a - 1, b) [ a >= 15 /\ a >= 1 ]
		(?, 1)    f2(a, b) -> f2(a - 1, b) [ 13 >= a /\ a >= 1 ]
		(1, 1)    f300(a, b) -> f2(a, b)
	start location:	f300
	leaf cost:	0

Testing for unsatisfiable constraints removes the following transition from problem 1:
	f2(a, b) -> f1(a - 1, c) [ a >= 15 /\ 0 >= a ]
We thus obtain the following problem:
2:	T:
		(?, 1)    f2(a, b) -> f1(c, d) [ 0 >= c + 1 /\ a = 14 ]
		(?, 1)    f2(a, b) -> f2(c, b) [ c >= 0 /\ a = 14 ]
		(?, 1)    f2(a, b) -> f1(a - 1, c) [ 13 >= a /\ 0 >= a ]
		(?, 1)    f2(a, b) -> f2(a - 1, b) [ a >= 15 /\ a >= 1 ]
		(?, 1)    f2(a, b) -> f2(a - 1, b) [ 13 >= a /\ a >= 1 ]
		(1, 1)    f300(a, b) -> f2(a, b)
	start location:	f300
	leaf cost:	0

Repeatedly removing leaves of the complexity graph in problem 2 produces the following problem:
3:	T:
		(?, 1)    f2(a, b) -> f2(c, b) [ c >= 0 /\ a = 14 ]
		(?, 1)    f2(a, b) -> f2(a - 1, b) [ a >= 15 /\ a >= 1 ]
		(?, 1)    f2(a, b) -> f2(a - 1, b) [ 13 >= a /\ a >= 1 ]
		(1, 1)    f300(a, b) -> f2(a, b)
	start location:	f300
	leaf cost:	2

Complexity upper bound ?

Time: 0.272 sec (SMT: 0.257 sec)