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

function testsBuild() {
    // Combine js files and append tests
    return gulp.src(getPaths().map(function(f) { return 'site/public/' + f; })) // Get rid of prepend
      .pipe(concat('tests/index.js'))
      .pipe(gap.prependFile('tests/prepend.js'))
      .pipe(gap.appendText('start();'))
      .pipe(addsrc.append(getTestPaths().map(function (f) { return 'tests/public/' + f; })))
      .pipe(concat('tests/index.js'))
      .pipe(gulp.dest('.'));
}

const compileRiot = () =>
    gulp.src('./src/riotTags/**')
    .pipe(riot({
        compact: false,
        template: 'pug'
    }))
    .pipe(concat('riot.js'))
    .pipe(gap.prependFile('./eslintfix.js'))
    .pipe(gulp.dest('./app/js'));

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'));
};