indexOfNonUnitaryGate() {
        for (let i = 0; i < this.gates.length; i++) {
            let gate = this.gates[i];
            if (gate !== undefined && !gate.isDefinitelyUnitary()) {
                return i;
            }
        }
        return undefined;
    }
@returns {undefined|!int} The index of some non-unitary gate in the column, if any.