Warning: CHC: Underflow (resulting value less than 0) happens here.
Counterexample:
arr = []
a = 0x0
x = 0
success = false

Transaction trace:
test.constructor()
State: arr = []
test.f(0x0, 0)
 --> input.sol:7:3:
  |
7 | 		--x;
  | 		^^^
