# modify method

dynamic modify (dynamic f(Set<String> s))

Helper method used to modify the set of css classes on this element.

f - callback with: s - a Set of all the css class name currently on this element.

After f returns, the modified set is written to the className property of this element.

## Implementation

    modify(f(Set<String> s)) {
      Set<String> s = readClasses();
      var ret = f(s);
      writeClasses(s);
      return ret;
    }