old_contents: 'use strict';
  
  var gulp = require('gulp');
  var sass = require('gulp-sass');
  var csso = require('gulp-csso');
  var nano = require('gulp-cssnano');
  
  gulp.task('sass', function () {
    return gulp.src('./styles/sass/**/style.scss')
      .pipe(sass().on('error', sass.logError))
      .pipe(gulp.dest('./styles/css/'));
  });
  
  gulp.task('sass:watch', function () {
    gulp.watch('./styles/sass/**/*.scss', ['sass']);
  });
  
  
  gulp.task('optimize', function () {
      return gulp.src('./styles/sass/**/style.scss')
          .pipe(sass())
          .pipe(nano())
          .pipe(csso())
          .pipe(gulp.dest('./styles/css'));
  });
  
new_contents: 'use strict';
  
  var gulp = require('gulp');
  var sass = require('gulp-sass');
  var csso = require('gulp-csso');
  var nano = require('gulp-cssnano');
  var purify = require('gulp-purifycss');
  
  gulp.task('pure', function() {
    return gulp.src('./styles/css/style.css')
      .pipe(purify(['./js/script.js', './index.html']))
      .pipe(gulp.dest('./styles/css/'));
  });
  
  gulp.task('sass', function () {
    return gulp.src('./styles/sass/**/style.scss')
      .pipe(sass().on('error', sass.logError))
      .pipe(gulp.dest('./styles/css/'));
  });
  
  gulp.task('sass:watch', function () {
    gulp.watch('./styles/sass/**/*.scss', ['sass']);
  });
  
  
  gulp.task('optimize', function () {
      return gulp.src('./styles/sass/**/style.scss')
          .pipe(sass())
          .pipe(purify(['./index.html']))
          .pipe(nano())
          .pipe(csso())
          .pipe(gulp.dest('./styles/css'));
  });
  
subject: Set up css purifier