How to use the @microsoft/web-library-build.parallel function in @microsoft/web-library-build

'use strict';

let build = require('@microsoft/web-library-build');
let serial = build.serial;
let parallel = build.parallel;
let buildConfig = build.getConfig();
let gulp = require('gulp');
let configFile = "./ftpconfig.json";
let fs = require('fs');
let path = require('path');
let del = require('del');
let gulpConnect = require('gulp-connect');

let isProduction = process.argv.indexOf('--production') >= 0;
let isNuke = process.argv.indexOf('nuke') >= 0;
let {
  libFolder,
  distFolder,
  packageFolder = ''
} = buildConfig;