fn assert_bool<I: Into<Self::Expr>>(&mut self, x: I) {
        let x = x.into();
        if x != F::zero() && x != F::one() {
            let backtrace = std::backtrace::Backtrace::force_capture();
            eprintln!("constraint failed: {x:?} is not a bool\n{backtrace}");
            panic!();
        }
    }
Assert that `x` is a boolean, i.e. either 0 or 1.
crates/stark/src/debug.rs