_processArguments() {
    const myParseArg = (argument, value, previous) => {
      // Extra processing for nice error message on parsing failure.
      let parsedValue = value;
      if (value !== null && argument.parseArg) {
        const invalidValueMessage = `error: command-argument value '${value}' is invalid for argument '${argument.name()}'.`;
        parsedValue = this._callParseArg(
          argument,
          value,
          previous,
          invalidValueMessage,
        );
      }
      return parsedValue;
    };

    this._checkNumberOfArguments();

    const processedArgs = [];
    this.registeredArguments.forEach((declaredArg, index) => {
      let value = declaredArg.defaultValue;
      if (declaredArg.variadic) {
        // Collect together remaining arguments for passing together as an array.
        if (index < this.args.length) {
          value = this.args.slice(index);
          if (declaredArg.parseArg) {
            value = value.reduce((processed, v) => {
              return myParseArg(declaredArg, v, processed);
            }, declaredArg.defaultValue);
          }
        } else if (value === undefined) {
          value = [];
        }
      } else if (index < this.args.length) {
        value = this.args[index];
        if (declaredArg.parseArg) {
          value = myParseArg(declaredArg, value, declaredArg.defaultValue);
        }
      }
      processedArgs[index] = value;
    });
    this.processedArgs = processedArgs;
  }
Process this.args using this.registeredArguments and save as this.processedArgs!

@private