_checkNumberOfArguments() {
    // too few
    this.registeredArguments.forEach((arg, i) => {
      if (arg.required && this.args[i] == null) {
        this.missingArgument(arg.name());
      }
    });
    // too many
    if (
      this.registeredArguments.length > 0 &&
      this.registeredArguments[this.registeredArguments.length - 1].variadic
    ) {
      return;
    }
    if (this.args.length > this.registeredArguments.length) {
      this._excessArguments(this.args);
    }
  }
Check this.args against expected this.registeredArguments.

@private