new_contents: from __future__ import with_statement
  
  from webassets.filter import ExternalTool
  from webassets.utils import working_directory
  
  
  class AutoprefixerFilter(ExternalTool):
      """Prefixes vendor-prefixes using `autoprefixer
      <https://github.com/ai/autoprefixer>`, which uses the `Can I Use?
      <http://www.caniuse.com>` database to know which prefixes need to be
      inserted.
  
      This depends on the `autoprefixer <https://github.com/ai/autoprefixer>`
      command line tool being installed (use ``npm install autoprefixer``).
  
      *Supported configuration options*:
  
      AUTOPREFIXER_BIN
          Path to the autoprefixer executable used to compile source files. By
          default, the filter will attempt to run ``autoprefixer`` via the
          system path.
  
      AUTOPREFIXER_BROWSERS
          The browser expressions to use.  This corresponds to the ``--browsers
          <value>`` flag, see the `--browsers documentation
          <https://github.com/ai/autoprefixer#browsers>`.  By default, this flag
          won't be passed, and autoprefixer's default will be used.
  
          Example::
  
              AUTOPREFIXER_BROWSERS = ['> 1%', 'last 2 versions', 'firefox 24', 'opera 12.1']
  
      AUTOPREFIXER_EXTRA_ARGS
          Additional options may be passed to ``autoprefixer`` using this
          setting, which expects a list of strings.
  
      """
      name = 'autoprefixer'
      options = {
          'autoprefixer': ('binary', 'AUTOPREFIXER_BIN'),
          'browsers': 'AUTOPREFIXER_BROWSERS',
          'extra_args': 'AUTOPREFIXER_EXTRA_ARGS',
      }
  
      def input(self, in_, out, source_path, **kw):
          # Set working directory to the source file so that includes are found
          args = [self.autoprefixer or 'autoprefixer']
          if self.browsers:
              if isinstance(self.browsers, (list, tuple)):
                  self.browsers = u','.join(self.browsers)
              args.extend(['--browsers', self.browsers])
          if self.extra_args:
              args.extend(self.extra_args)
          with working_directory(filename=source_path):
              self.subprocess(args, out, in_)
  
subject: Add autoprefixer support.