docs: function unifiedDiff(err, escape) {
    var indent = '      ';
    function cleanUp(line) {
      if (escape) {
        line = escapeInvisibles(line);
      }
      if (line[0] === '+') return indent + colorLines('diff added', line);
      if (line[0] === '-') return indent + colorLines('diff removed', line);
      if (line.match(/\@\@/)) return null;
      if (line.match(/\\ No newline/)) return null;
      else return indent + line;
    }
    function notBlank(line) {
      return line != null;
    }
    msg = diff.createPatch('string', err.actual, err.expected);
    var lines = msg.split('\n').splice(4);
    return '\n      '
           + colorLines('diff added',   '+ expected') + ' '
           + colorLines('diff removed', '- actual')
           + '\n\n'
           + lines.map(cleanUp).filter(notBlank).join('\n');
  }