Warning: CHC: Out of bounds access happens here.
Counterexample:
arr = []
a = 0x0
x = 0
success = true

Transaction trace:
test.constructor()
State: arr = []
test.f(0x0, 1)
    a.call{value: x}("") -- untrusted external call
  --> input.sol:14:3:
   |
14 | 		arr[x];
   | 		^^^^^^
