function getForBoolean () {
    if (dataFrom === undefined) {
      from[attribute] = false;
    } else {
      from[attribute] = strToBool(dataFrom);
    }
    from[attribute] = boolToNum(from[attribute]);
    to[attribute] = boolToNum(strToBool(dataTo));
    partialSetAttribute = function (value) {
      el.setAttribute(attribute, !!value[attribute]);
    };
  }
Animation a boolean (e.g., visible).
Have to convert from boolean to an integer (0 is false, > 0 is true) for tween.