docs: function errorDiff(err, type, escape) {
    var actual   = escape ? escapeInvisibles(err.actual)   : err.actual;
    var expected = escape ? escapeInvisibles(err.expected) : err.expected;
    return diff['diff' + type](actual, expected).map(function(str){
      if (str.added) return colorLines('diff added', str.value);
      if (str.removed) return colorLines('diff removed', str.value);
      return str.value;
    }).join('');
  }