Live demo

Edit the JSON and ask Z3. Calls the real public route.

The public route runs the 2026-11-02 engine. The fourth example shows that day's fix: the answer names the failure (function_raises) instead of reporting an ordinary violation. Re-checked live on 2026-11-03.

A real bug

Expected: contract_holds: false, counterexample price = 0 (discount(0) is -5).

A proof

Expected: contract_holds: true: for every integer.

Python's modulo, not the solver's

Expected: contract_holds: true: in Python x % -3 is 0, -1 or -2.

Division by zero

Expected: contract_holds: false, failure: function_raises, count = 0.

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