code: function isA(elt, set) {
    if (typeof set === 'string') {
      // convenience case for testing a particular HTML element
      return elt.namespaceURI === NAMESPACE.HTML &&
        elt.localName === set;
    }
    var tagnames = set[elt.namespaceURI];
    return tagnames && tagnames[elt.localName];
  }
docs: Determine whether the element is a member of the set. The set is an object that maps namespaces to objects. The objects then map local tagnames to the value true if that tag is part of the set