How to use the gulp-append-prepend.appendFile function in gulp-append-prepend

gulp.task('debug-vendor-css', false, ["init-vendor-css-file"], function () {
    // makes all in one file
    return gulp.src(OUT_DIR + "/vendor/vendor-debug.css")
        .pipe(gap.prependText(VENDOR_CSS_EXTRA_PARAMS))
        .pipe(gap.appendFile(ALL_VENDOR_CSS))
        .pipe(gulp.dest(OUT_DIR + "/vendor/"));
});

gulp.task('styles:rtl', function () {
    gulp.src(config.src.styles.all)
        .pipe(sourcemaps.init())
        .pipe(sass().on('error', sass.logError))
        .pipe(prefix('last 1 version'))
        .pipe(gulpif(!config.dev, csso()))
        .pipe(gap.appendFile('themes/rtl.css'))
        .pipe(rename({
            dirname: "",
            prefix:"rtl_"
        }))
        .pipe(gulp.dest(config.dest.themes))
        .pipe(gulpif(config.build, gulp.dest(config.dist.storybook + config.dest.themes)))
        .pipe(gulpif(config.dev, reload({stream:true})));
});

function build() {
  return gulp.src(paths.includes)
    .pipe(replace('define([', function() {
      var pathObj = path.parse(path.relative('src', this.file.path));
      var name = path.format(Object.assign({}, pathObj, { ext: null, base: null }));
      return 'define(\'' + name + '\', [';
    }))
    .pipe(concat('select2.multi-checkboxes.js', { newLine: '\n\n' }))
    .pipe(indent())
    .pipe(gap.prependFile('build/wrapper.start.js', { trim: false }))
    .pipe(gap.appendFile('build/wrapper.end.js', { trim: false, separator: '' }))
    .pipe(gulp.dest('build'));
};