docs: function processFiles(filenames = [], options = defaultOptions) {
  
      if (options.convertUrls) {
          options.target = resolve(process.cwd(), options.convertUrls).split(PATH_SEP)
      }
  
      const uglies = []
  
      // process files
      filenames.forEach(filename => {
          try {
              const content = readFileSync(filename, 'utf8')
              if (content.length) {
                  if (options.convertUrls) {
                      options.source = resolve(process.cwd(), filename).split(PATH_SEP)
                      options.source.pop()
                  }
                  uglies.push(processString(content, options))
              }
          } catch (e) {
              if (options.debug) {
                  console.error(`uglifycss: unable to process "${filename}"\n${e.stack}`)
              } else {
                  console.error(`uglifycss: unable to process "${filename}"\n\t${e}`)
              }
              process.exit(1)
          }
      })
  
      // return concat'd results
      return uglies.join('')
  }