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

Transaction trace:
test.constructor()
State: arr = []
test.f(0, 0)
 --> model_checker_targets_underflow_chc/input.sol:8:3:
  |
8 | 		--x;
  | 		^^^