getTextFile(filename: string) {
  // The Observable returned by get() is of type Observable
  // because a text response was specified.
  // There's no need to pass a  type parameter to get().
  return this.http.get(filename, {responseType: 'text'})
    .pipe(
      tap( // Log the result or error
        data => this.log(filename, data),
        error => this.logError(filename, error)
      )
    );
}