withInputSetToConstant(letter, value) {
        let r = this._clone();
        r.customContextFromGates = new Map(r.customContextFromGates);
        r.customContextFromGates.delete(`Input Range ${letter}`);
        r.customContextFromGates.set(`Input Default ${letter}`, value);
        return r;
    }
@param {!string} letter
@param {!int} value
@returns {!CircuitEvalContext}