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