function processCSS(processor, input, output, fn) {
  function doProcess(css, fn) {
    function onResult(result) {
      if (typeof result.warnings === 'function') {
        result.warnings().forEach(console.error);
      }

      fn(null, result.css);
    }

    var result = processor.process(css, {
      safe: argv.safe,
      from: input,
      to: output
    });

    if (typeof result.then === 'function') {
      result.then(onResult).catch(fn);
    } else{
      process.nextTick(onResult.bind(null, result));
    }
  }

  async.waterfall([
    async.apply(readFile, input),
    doProcess,
    async.apply(writeFile, output)
  ], fn);
}