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gulp.task('sass:production', function() {
  return gulp.src(config.sass.src)
    .pipe(sass({
      includePaths: config.sass.sassPaths,
      outputStyle: 'expanded'
    })
    .on('error', sass.logError))
    .pipe(uncss({
    html: glob.sync(config.optimize.uncss.src),
    ignore: [
      /\.fade/,
      /\.modal/,
      /\.affix/,
      /\.tooltip/,
      /\.popover/,
      /\.collaps/,
      /\.carousel-inner/,
      /\.open/,
      /\.in/,
      /\.flipped/,
      /\.waves/,
      /(#|\.)(is-)/,
      /(#|\.)(has-)/,
      /(#|\.)(js-)/,
      // sample regex to ignore '.active' class
      /\.\bactive\b/
    ],
    ignoreSheets: [
      // ignoreSheets: sample regex to ignore Google fonts
      /fonts.googleapis/
    ]
    }))
    .pipe(prefix({
      browsers: ['last 2 versions', 'ie >= 9']
    }))
    .pipe(minifycss(config.optimize.css.options))
    .pipe(gulp.dest(config.sass.prod))
});

// More importantly, this invocation of UnCSS reads the stylesheets linked in index.html to find the correct CSS-file, then processes it.