MAYBE

Initial complexity problem:
1:	T:
		(?, 1)    f0(a, b, c, d) -> f3(a, e, c, d) [ a >= 10 ]
		(?, 1)    f0(a, b, c, d) -> f0(a - 1, b, a, d) [ 9 >= a ]
		(?, 1)    f1(a, b, c, d) -> f0(-1, b, c, -1) [ 9 >= a ]
		(1, 1)    f2(a, b, c, d) -> f0(0, b, c, d)
	start location:	f2
	leaf cost:	0

Repeatedly removing leaves of the complexity graph in problem 1 produces the following problem:
2:	T:
		(?, 1)    f0(a, b, c, d) -> f0(a - 1, b, a, d) [ 9 >= a ]
		(?, 1)    f1(a, b, c, d) -> f0(-1, b, c, -1) [ 9 >= a ]
		(1, 1)    f2(a, b, c, d) -> f0(0, b, c, d)
	start location:	f2
	leaf cost:	1

Testing for reachability in the complexity graph removes the following transition from problem 2:
	f1(a, b, c, d) -> f0(-1, b, c, -1) [ 9 >= a ]
We thus obtain the following problem:
3:	T:
		(?, 1)    f0(a, b, c, d) -> f0(a - 1, b, a, d) [ 9 >= a ]
		(1, 1)    f2(a, b, c, d) -> f0(0, b, c, d)
	start location:	f2
	leaf cost:	1

Applied AI with 'oct' on problem 3 to obtain the following invariants:
  For symbol f0: -X_1 >= 0


This yielded the following problem:
4:	T:
		(1, 1)    f2(a, b, c, d) -> f0(0, b, c, d)
		(?, 1)    f0(a, b, c, d) -> f0(a - 1, b, a, d) [ -a >= 0 /\ 9 >= a ]
	start location:	f2
	leaf cost:	1

By chaining the transition f2(a, b, c, d) -> f0(0, b, c, d) with all transitions in problem 4, the following new transition is obtained:
	f2(a, b, c, d) -> f0(-1, b, 0, d) [ 0 >= 0 /\ 9 >= 0 ]
We thus obtain the following problem:
5:	T:
		(1, 2)    f2(a, b, c, d) -> f0(-1, b, 0, d) [ 0 >= 0 /\ 9 >= 0 ]
		(?, 1)    f0(a, b, c, d) -> f0(a - 1, b, a, d) [ -a >= 0 /\ 9 >= a ]
	start location:	f2
	leaf cost:	1

By chaining the transition f2(a, b, c, d) -> f0(-1, b, 0, d) [ 0 >= 0 /\ 9 >= 0 ] with all transitions in problem 5, the following new transition is obtained:
	f2(a, b, c, d) -> f0(-2, b, -1, d) [ 0 >= 0 /\ 9 >= 0 /\ 1 >= 0 /\ 9 >= -1 ]
We thus obtain the following problem:
6:	T:
		(1, 3)    f2(a, b, c, d) -> f0(-2, b, -1, d) [ 0 >= 0 /\ 9 >= 0 /\ 1 >= 0 /\ 9 >= -1 ]
		(?, 1)    f0(a, b, c, d) -> f0(a - 1, b, a, d) [ -a >= 0 /\ 9 >= a ]
	start location:	f2
	leaf cost:	1

By chaining the transition f2(a, b, c, d) -> f0(-2, b, -1, d) [ 0 >= 0 /\ 9 >= 0 /\ 1 >= 0 /\ 9 >= -1 ] with all transitions in problem 6, the following new transition is obtained:
	f2(a, b, c, d) -> f0(-3, b, -2, d) [ 0 >= 0 /\ 9 >= 0 /\ 1 >= 0 /\ 9 >= -1 /\ 2 >= 0 /\ 9 >= -2 ]
We thus obtain the following problem:
7:	T:
		(1, 4)    f2(a, b, c, d) -> f0(-3, b, -2, d) [ 0 >= 0 /\ 9 >= 0 /\ 1 >= 0 /\ 9 >= -1 /\ 2 >= 0 /\ 9 >= -2 ]
		(?, 1)    f0(a, b, c, d) -> f0(a - 1, b, a, d) [ -a >= 0 /\ 9 >= a ]
	start location:	f2
	leaf cost:	1

By chaining the transition f2(a, b, c, d) -> f0(-3, b, -2, d) [ 0 >= 0 /\ 9 >= 0 /\ 1 >= 0 /\ 9 >= -1 /\ 2 >= 0 /\ 9 >= -2 ] with all transitions in problem 7, the following new transition is obtained:
	f2(a, b, c, d) -> f0(-4, b, -3, d) [ 0 >= 0 /\ 9 >= 0 /\ 1 >= 0 /\ 9 >= -1 /\ 2 >= 0 /\ 9 >= -2 /\ 3 >= 0 /\ 9 >= -3 ]
We thus obtain the following problem:
8:	T:
		(1, 5)    f2(a, b, c, d) -> f0(-4, b, -3, d) [ 0 >= 0 /\ 9 >= 0 /\ 1 >= 0 /\ 9 >= -1 /\ 2 >= 0 /\ 9 >= -2 /\ 3 >= 0 /\ 9 >= -3 ]
		(?, 1)    f0(a, b, c, d) -> f0(a - 1, b, a, d) [ -a >= 0 /\ 9 >= a ]
	start location:	f2
	leaf cost:	1

By chaining the transition f2(a, b, c, d) -> f0(-4, b, -3, d) [ 0 >= 0 /\ 9 >= 0 /\ 1 >= 0 /\ 9 >= -1 /\ 2 >= 0 /\ 9 >= -2 /\ 3 >= 0 /\ 9 >= -3 ] with all transitions in problem 8, the following new transition is obtained:
	f2(a, b, c, d) -> f0(-5, b, -4, d) [ 0 >= 0 /\ 9 >= 0 /\ 1 >= 0 /\ 9 >= -1 /\ 2 >= 0 /\ 9 >= -2 /\ 3 >= 0 /\ 9 >= -3 /\ 4 >= 0 /\ 9 >= -4 ]
We thus obtain the following problem:
9:	T:
		(1, 6)    f2(a, b, c, d) -> f0(-5, b, -4, d) [ 0 >= 0 /\ 9 >= 0 /\ 1 >= 0 /\ 9 >= -1 /\ 2 >= 0 /\ 9 >= -2 /\ 3 >= 0 /\ 9 >= -3 /\ 4 >= 0 /\ 9 >= -4 ]
		(?, 1)    f0(a, b, c, d) -> f0(a - 1, b, a, d) [ -a >= 0 /\ 9 >= a ]
	start location:	f2
	leaf cost:	1

By chaining the transition f2(a, b, c, d) -> f0(-5, b, -4, d) [ 0 >= 0 /\ 9 >= 0 /\ 1 >= 0 /\ 9 >= -1 /\ 2 >= 0 /\ 9 >= -2 /\ 3 >= 0 /\ 9 >= -3 /\ 4 >= 0 /\ 9 >= -4 ] with all transitions in problem 9, the following new transition is obtained:
	f2(a, b, c, d) -> f0(-6, b, -5, d) [ 0 >= 0 /\ 9 >= 0 /\ 1 >= 0 /\ 9 >= -1 /\ 2 >= 0 /\ 9 >= -2 /\ 3 >= 0 /\ 9 >= -3 /\ 4 >= 0 /\ 9 >= -4 /\ 5 >= 0 /\ 9 >= -5 ]
We thus obtain the following problem:
10:	T:
		(1, 7)    f2(a, b, c, d) -> f0(-6, b, -5, d) [ 0 >= 0 /\ 9 >= 0 /\ 1 >= 0 /\ 9 >= -1 /\ 2 >= 0 /\ 9 >= -2 /\ 3 >= 0 /\ 9 >= -3 /\ 4 >= 0 /\ 9 >= -4 /\ 5 >= 0 /\ 9 >= -5 ]
		(?, 1)    f0(a, b, c, d) -> f0(a - 1, b, a, d) [ -a >= 0 /\ 9 >= a ]
	start location:	f2
	leaf cost:	1

By chaining the transition f2(a, b, c, d) -> f0(-6, b, -5, d) [ 0 >= 0 /\ 9 >= 0 /\ 1 >= 0 /\ 9 >= -1 /\ 2 >= 0 /\ 9 >= -2 /\ 3 >= 0 /\ 9 >= -3 /\ 4 >= 0 /\ 9 >= -4 /\ 5 >= 0 /\ 9 >= -5 ] with all transitions in problem 10, the following new transition is obtained:
	f2(a, b, c, d) -> f0(-7, b, -6, d) [ 0 >= 0 /\ 9 >= 0 /\ 1 >= 0 /\ 9 >= -1 /\ 2 >= 0 /\ 9 >= -2 /\ 3 >= 0 /\ 9 >= -3 /\ 4 >= 0 /\ 9 >= -4 /\ 5 >= 0 /\ 9 >= -5 /\ 6 >= 0 /\ 9 >= -6 ]
We thus obtain the following problem:
11:	T:
		(1, 8)    f2(a, b, c, d) -> f0(-7, b, -6, d) [ 0 >= 0 /\ 9 >= 0 /\ 1 >= 0 /\ 9 >= -1 /\ 2 >= 0 /\ 9 >= -2 /\ 3 >= 0 /\ 9 >= -3 /\ 4 >= 0 /\ 9 >= -4 /\ 5 >= 0 /\ 9 >= -5 /\ 6 >= 0 /\ 9 >= -6 ]
		(?, 1)    f0(a, b, c, d) -> f0(a - 1, b, a, d) [ -a >= 0 /\ 9 >= a ]
	start location:	f2
	leaf cost:	1

By chaining the transition f2(a, b, c, d) -> f0(-7, b, -6, d) [ 0 >= 0 /\ 9 >= 0 /\ 1 >= 0 /\ 9 >= -1 /\ 2 >= 0 /\ 9 >= -2 /\ 3 >= 0 /\ 9 >= -3 /\ 4 >= 0 /\ 9 >= -4 /\ 5 >= 0 /\ 9 >= -5 /\ 6 >= 0 /\ 9 >= -6 ] with all transitions in problem 11, the following new transition is obtained:
	f2(a, b, c, d) -> f0(-8, b, -7, d) [ 0 >= 0 /\ 9 >= 0 /\ 1 >= 0 /\ 9 >= -1 /\ 2 >= 0 /\ 9 >= -2 /\ 3 >= 0 /\ 9 >= -3 /\ 4 >= 0 /\ 9 >= -4 /\ 5 >= 0 /\ 9 >= -5 /\ 6 >= 0 /\ 9 >= -6 /\ 7 >= 0 /\ 9 >= -7 ]
We thus obtain the following problem:
12:	T:
		(1, 9)    f2(a, b, c, d) -> f0(-8, b, -7, d) [ 0 >= 0 /\ 9 >= 0 /\ 1 >= 0 /\ 9 >= -1 /\ 2 >= 0 /\ 9 >= -2 /\ 3 >= 0 /\ 9 >= -3 /\ 4 >= 0 /\ 9 >= -4 /\ 5 >= 0 /\ 9 >= -5 /\ 6 >= 0 /\ 9 >= -6 /\ 7 >= 0 /\ 9 >= -7 ]
		(?, 1)    f0(a, b, c, d) -> f0(a - 1, b, a, d) [ -a >= 0 /\ 9 >= a ]
	start location:	f2
	leaf cost:	1

By chaining the transition f2(a, b, c, d) -> f0(-8, b, -7, d) [ 0 >= 0 /\ 9 >= 0 /\ 1 >= 0 /\ 9 >= -1 /\ 2 >= 0 /\ 9 >= -2 /\ 3 >= 0 /\ 9 >= -3 /\ 4 >= 0 /\ 9 >= -4 /\ 5 >= 0 /\ 9 >= -5 /\ 6 >= 0 /\ 9 >= -6 /\ 7 >= 0 /\ 9 >= -7 ] with all transitions in problem 12, the following new transition is obtained:
	f2(a, b, c, d) -> f0(-9, b, -8, d) [ 0 >= 0 /\ 9 >= 0 /\ 1 >= 0 /\ 9 >= -1 /\ 2 >= 0 /\ 9 >= -2 /\ 3 >= 0 /\ 9 >= -3 /\ 4 >= 0 /\ 9 >= -4 /\ 5 >= 0 /\ 9 >= -5 /\ 6 >= 0 /\ 9 >= -6 /\ 7 >= 0 /\ 9 >= -7 /\ 8 >= 0 /\ 9 >= -8 ]
We thus obtain the following problem:
13:	T:
		(1, 10)    f2(a, b, c, d) -> f0(-9, b, -8, d) [ 0 >= 0 /\ 9 >= 0 /\ 1 >= 0 /\ 9 >= -1 /\ 2 >= 0 /\ 9 >= -2 /\ 3 >= 0 /\ 9 >= -3 /\ 4 >= 0 /\ 9 >= -4 /\ 5 >= 0 /\ 9 >= -5 /\ 6 >= 0 /\ 9 >= -6 /\ 7 >= 0 /\ 9 >= -7 /\ 8 >= 0 /\ 9 >= -8 ]
		(?, 1)     f0(a, b, c, d) -> f0(a - 1, b, a, d) [ -a >= 0 /\ 9 >= a ]
	start location:	f2
	leaf cost:	1

By chaining the transition f2(a, b, c, d) -> f0(-9, b, -8, d) [ 0 >= 0 /\ 9 >= 0 /\ 1 >= 0 /\ 9 >= -1 /\ 2 >= 0 /\ 9 >= -2 /\ 3 >= 0 /\ 9 >= -3 /\ 4 >= 0 /\ 9 >= -4 /\ 5 >= 0 /\ 9 >= -5 /\ 6 >= 0 /\ 9 >= -6 /\ 7 >= 0 /\ 9 >= -7 /\ 8 >= 0 /\ 9 >= -8 ] with all transitions in problem 13, the following new transition is obtained:
	f2(a, b, c, d) -> f0(-10, b, -9, d) [ 0 >= 0 /\ 9 >= 0 /\ 1 >= 0 /\ 9 >= -1 /\ 2 >= 0 /\ 9 >= -2 /\ 3 >= 0 /\ 9 >= -3 /\ 4 >= 0 /\ 9 >= -4 /\ 5 >= 0 /\ 9 >= -5 /\ 6 >= 0 /\ 9 >= -6 /\ 7 >= 0 /\ 9 >= -7 /\ 8 >= 0 /\ 9 >= -8 /\ 9 >= -9 ]
We thus obtain the following problem:
14:	T:
		(1, 11)    f2(a, b, c, d) -> f0(-10, b, -9, d) [ 0 >= 0 /\ 9 >= 0 /\ 1 >= 0 /\ 9 >= -1 /\ 2 >= 0 /\ 9 >= -2 /\ 3 >= 0 /\ 9 >= -3 /\ 4 >= 0 /\ 9 >= -4 /\ 5 >= 0 /\ 9 >= -5 /\ 6 >= 0 /\ 9 >= -6 /\ 7 >= 0 /\ 9 >= -7 /\ 8 >= 0 /\ 9 >= -8 /\ 9 >= -9 ]
		(?, 1)     f0(a, b, c, d) -> f0(a - 1, b, a, d) [ -a >= 0 /\ 9 >= a ]
	start location:	f2
	leaf cost:	1

By chaining the transition f2(a, b, c, d) -> f0(-10, b, -9, d) [ 0 >= 0 /\ 9 >= 0 /\ 1 >= 0 /\ 9 >= -1 /\ 2 >= 0 /\ 9 >= -2 /\ 3 >= 0 /\ 9 >= -3 /\ 4 >= 0 /\ 9 >= -4 /\ 5 >= 0 /\ 9 >= -5 /\ 6 >= 0 /\ 9 >= -6 /\ 7 >= 0 /\ 9 >= -7 /\ 8 >= 0 /\ 9 >= -8 /\ 9 >= -9 ] with all transitions in problem 14, the following new transition is obtained:
	f2(a, b, c, d) -> f0(-11, b, -10, d) [ 0 >= 0 /\ 9 >= 0 /\ 1 >= 0 /\ 9 >= -1 /\ 2 >= 0 /\ 9 >= -2 /\ 3 >= 0 /\ 9 >= -3 /\ 4 >= 0 /\ 9 >= -4 /\ 5 >= 0 /\ 9 >= -5 /\ 6 >= 0 /\ 9 >= -6 /\ 7 >= 0 /\ 9 >= -7 /\ 8 >= 0 /\ 9 >= -8 /\ 9 >= -9 /\ 10 >= 0 /\ 9 >= -10 ]
We thus obtain the following problem:
15:	T:
		(1, 12)    f2(a, b, c, d) -> f0(-11, b, -10, d) [ 0 >= 0 /\ 9 >= 0 /\ 1 >= 0 /\ 9 >= -1 /\ 2 >= 0 /\ 9 >= -2 /\ 3 >= 0 /\ 9 >= -3 /\ 4 >= 0 /\ 9 >= -4 /\ 5 >= 0 /\ 9 >= -5 /\ 6 >= 0 /\ 9 >= -6 /\ 7 >= 0 /\ 9 >= -7 /\ 8 >= 0 /\ 9 >= -8 /\ 9 >= -9 /\ 10 >= 0 /\ 9 >= -10 ]
		(?, 1)     f0(a, b, c, d) -> f0(a - 1, b, a, d) [ -a >= 0 /\ 9 >= a ]
	start location:	f2
	leaf cost:	1

By chaining the transition f2(a, b, c, d) -> f0(-11, b, -10, d) [ 0 >= 0 /\ 9 >= 0 /\ 1 >= 0 /\ 9 >= -1 /\ 2 >= 0 /\ 9 >= -2 /\ 3 >= 0 /\ 9 >= -3 /\ 4 >= 0 /\ 9 >= -4 /\ 5 >= 0 /\ 9 >= -5 /\ 6 >= 0 /\ 9 >= -6 /\ 7 >= 0 /\ 9 >= -7 /\ 8 >= 0 /\ 9 >= -8 /\ 9 >= -9 /\ 10 >= 0 /\ 9 >= -10 ] with all transitions in problem 15, the following new transition is obtained:
	f2(a, b, c, d) -> f0(-12, b, -11, d) [ 0 >= 0 /\ 9 >= 0 /\ 1 >= 0 /\ 9 >= -1 /\ 2 >= 0 /\ 9 >= -2 /\ 3 >= 0 /\ 9 >= -3 /\ 4 >= 0 /\ 9 >= -4 /\ 5 >= 0 /\ 9 >= -5 /\ 6 >= 0 /\ 9 >= -6 /\ 7 >= 0 /\ 9 >= -7 /\ 8 >= 0 /\ 9 >= -8 /\ 9 >= -9 /\ 10 >= 0 /\ 9 >= -10 /\ 11 >= 0 /\ 9 >= -11 ]
We thus obtain the following problem:
16:	T:
		(1, 13)    f2(a, b, c, d) -> f0(-12, b, -11, d) [ 0 >= 0 /\ 9 >= 0 /\ 1 >= 0 /\ 9 >= -1 /\ 2 >= 0 /\ 9 >= -2 /\ 3 >= 0 /\ 9 >= -3 /\ 4 >= 0 /\ 9 >= -4 /\ 5 >= 0 /\ 9 >= -5 /\ 6 >= 0 /\ 9 >= -6 /\ 7 >= 0 /\ 9 >= -7 /\ 8 >= 0 /\ 9 >= -8 /\ 9 >= -9 /\ 10 >= 0 /\ 9 >= -10 /\ 11 >= 0 /\ 9 >= -11 ]
		(?, 1)     f0(a, b, c, d) -> f0(a - 1, b, a, d) [ -a >= 0 /\ 9 >= a ]
	start location:	f2
	leaf cost:	1

By chaining the transition f2(a, b, c, d) -> f0(-12, b, -11, d) [ 0 >= 0 /\ 9 >= 0 /\ 1 >= 0 /\ 9 >= -1 /\ 2 >= 0 /\ 9 >= -2 /\ 3 >= 0 /\ 9 >= -3 /\ 4 >= 0 /\ 9 >= -4 /\ 5 >= 0 /\ 9 >= -5 /\ 6 >= 0 /\ 9 >= -6 /\ 7 >= 0 /\ 9 >= -7 /\ 8 >= 0 /\ 9 >= -8 /\ 9 >= -9 /\ 10 >= 0 /\ 9 >= -10 /\ 11 >= 0 /\ 9 >= -11 ] with all transitions in problem 16, the following new transition is obtained:
	f2(a, b, c, d) -> f0(-13, b, -12, d) [ 0 >= 0 /\ 9 >= 0 /\ 1 >= 0 /\ 9 >= -1 /\ 2 >= 0 /\ 9 >= -2 /\ 3 >= 0 /\ 9 >= -3 /\ 4 >= 0 /\ 9 >= -4 /\ 5 >= 0 /\ 9 >= -5 /\ 6 >= 0 /\ 9 >= -6 /\ 7 >= 0 /\ 9 >= -7 /\ 8 >= 0 /\ 9 >= -8 /\ 9 >= -9 /\ 10 >= 0 /\ 9 >= -10 /\ 11 >= 0 /\ 9 >= -11 /\ 12 >= 0 /\ 9 >= -12 ]
We thus obtain the following problem:
17:	T:
		(1, 14)    f2(a, b, c, d) -> f0(-13, b, -12, d) [ 0 >= 0 /\ 9 >= 0 /\ 1 >= 0 /\ 9 >= -1 /\ 2 >= 0 /\ 9 >= -2 /\ 3 >= 0 /\ 9 >= -3 /\ 4 >= 0 /\ 9 >= -4 /\ 5 >= 0 /\ 9 >= -5 /\ 6 >= 0 /\ 9 >= -6 /\ 7 >= 0 /\ 9 >= -7 /\ 8 >= 0 /\ 9 >= -8 /\ 9 >= -9 /\ 10 >= 0 /\ 9 >= -10 /\ 11 >= 0 /\ 9 >= -11 /\ 12 >= 0 /\ 9 >= -12 ]
		(?, 1)     f0(a, b, c, d) -> f0(a - 1, b, a, d) [ -a >= 0 /\ 9 >= a ]
	start location:	f2
	leaf cost:	1

By chaining the transition f2(a, b, c, d) -> f0(-13, b, -12, d) [ 0 >= 0 /\ 9 >= 0 /\ 1 >= 0 /\ 9 >= -1 /\ 2 >= 0 /\ 9 >= -2 /\ 3 >= 0 /\ 9 >= -3 /\ 4 >= 0 /\ 9 >= -4 /\ 5 >= 0 /\ 9 >= -5 /\ 6 >= 0 /\ 9 >= -6 /\ 7 >= 0 /\ 9 >= -7 /\ 8 >= 0 /\ 9 >= -8 /\ 9 >= -9 /\ 10 >= 0 /\ 9 >= -10 /\ 11 >= 0 /\ 9 >= -11 /\ 12 >= 0 /\ 9 >= -12 ] with all transitions in problem 17, the following new transition is obtained:
	f2(a, b, c, d) -> f0(-14, b, -13, d) [ 0 >= 0 /\ 9 >= 0 /\ 1 >= 0 /\ 9 >= -1 /\ 2 >= 0 /\ 9 >= -2 /\ 3 >= 0 /\ 9 >= -3 /\ 4 >= 0 /\ 9 >= -4 /\ 5 >= 0 /\ 9 >= -5 /\ 6 >= 0 /\ 9 >= -6 /\ 7 >= 0 /\ 9 >= -7 /\ 8 >= 0 /\ 9 >= -8 /\ 9 >= -9 /\ 10 >= 0 /\ 9 >= -10 /\ 11 >= 0 /\ 9 >= -11 /\ 12 >= 0 /\ 9 >= -12 /\ 13 >= 0 /\ 9 >= -13 ]
We thus obtain the following problem:
18:	T:
		(1, 15)    f2(a, b, c, d) -> f0(-14, b, -13, d) [ 0 >= 0 /\ 9 >= 0 /\ 1 >= 0 /\ 9 >= -1 /\ 2 >= 0 /\ 9 >= -2 /\ 3 >= 0 /\ 9 >= -3 /\ 4 >= 0 /\ 9 >= -4 /\ 5 >= 0 /\ 9 >= -5 /\ 6 >= 0 /\ 9 >= -6 /\ 7 >= 0 /\ 9 >= -7 /\ 8 >= 0 /\ 9 >= -8 /\ 9 >= -9 /\ 10 >= 0 /\ 9 >= -10 /\ 11 >= 0 /\ 9 >= -11 /\ 12 >= 0 /\ 9 >= -12 /\ 13 >= 0 /\ 9 >= -13 ]
		(?, 1)     f0(a, b, c, d) -> f0(a - 1, b, a, d) [ -a >= 0 /\ 9 >= a ]
	start location:	f2
	leaf cost:	1

By chaining the transition f2(a, b, c, d) -> f0(-14, b, -13, d) [ 0 >= 0 /\ 9 >= 0 /\ 1 >= 0 /\ 9 >= -1 /\ 2 >= 0 /\ 9 >= -2 /\ 3 >= 0 /\ 9 >= -3 /\ 4 >= 0 /\ 9 >= -4 /\ 5 >= 0 /\ 9 >= -5 /\ 6 >= 0 /\ 9 >= -6 /\ 7 >= 0 /\ 9 >= -7 /\ 8 >= 0 /\ 9 >= -8 /\ 9 >= -9 /\ 10 >= 0 /\ 9 >= -10 /\ 11 >= 0 /\ 9 >= -11 /\ 12 >= 0 /\ 9 >= -12 /\ 13 >= 0 /\ 9 >= -13 ] with all transitions in problem 18, the following new transition is obtained:
	f2(a, b, c, d) -> f0(-15, b, -14, d) [ 0 >= 0 /\ 9 >= 0 /\ 1 >= 0 /\ 9 >= -1 /\ 2 >= 0 /\ 9 >= -2 /\ 3 >= 0 /\ 9 >= -3 /\ 4 >= 0 /\ 9 >= -4 /\ 5 >= 0 /\ 9 >= -5 /\ 6 >= 0 /\ 9 >= -6 /\ 7 >= 0 /\ 9 >= -7 /\ 8 >= 0 /\ 9 >= -8 /\ 9 >= -9 /\ 10 >= 0 /\ 9 >= -10 /\ 11 >= 0 /\ 9 >= -11 /\ 12 >= 0 /\ 9 >= -12 /\ 13 >= 0 /\ 9 >= -13 /\ 14 >= 0 /\ 9 >= -14 ]
We thus obtain the following problem:
19:	T:
		(1, 16)    f2(a, b, c, d) -> f0(-15, b, -14, d) [ 0 >= 0 /\ 9 >= 0 /\ 1 >= 0 /\ 9 >= -1 /\ 2 >= 0 /\ 9 >= -2 /\ 3 >= 0 /\ 9 >= -3 /\ 4 >= 0 /\ 9 >= -4 /\ 5 >= 0 /\ 9 >= -5 /\ 6 >= 0 /\ 9 >= -6 /\ 7 >= 0 /\ 9 >= -7 /\ 8 >= 0 /\ 9 >= -8 /\ 9 >= -9 /\ 10 >= 0 /\ 9 >= -10 /\ 11 >= 0 /\ 9 >= -11 /\ 12 >= 0 /\ 9 >= -12 /\ 13 >= 0 /\ 9 >= -13 /\ 14 >= 0 /\ 9 >= -14 ]
		(?, 1)     f0(a, b, c, d) -> f0(a - 1, b, a, d) [ -a >= 0 /\ 9 >= a ]
	start location:	f2
	leaf cost:	1

Complexity upper bound ?

Time: 0.853 sec (SMT: 0.791 sec)