lookupFunctionArgumentBitRatio(functionName, argumentName) {
    if (!this._isFunction(functionName)) {
      throw new Error('function not found');
    }
    if (this.rootNode.name === functionName) {
      const i = this.rootNode.argumentNames.indexOf(argumentName);
      if (i !== -1) {
        return this.rootNode.argumentBitRatios[i];
      }
    }
    const node = this._getFunction(functionName);
    const i = node.argumentNames.indexOf(argumentName);
    if (i === -1) {
      throw new Error('argument not found');
    }
    const bitRatio = node.argumentBitRatios[i];
    if (typeof bitRatio !== 'number') {
      throw new Error('argument bit ratio not found');
    }
    return bitRatio;
  }
@param {string} functionName
@param {string} argumentName
@return {number}