public final XSComponent selectSingle(XSComponent contextNode){
  Iterator<XSComponent> r=select(Iterators.singleton(contextNode));
  if (r.hasNext())   return r.next();
  return null;
}

public final XSComponent selectSingle ( XSComponent contextNode ) { Iterator < XSComponent > r = select ( Iterators . singleton ( contextNode ) ) ; if ( r . hasNext ( ) ) return r . next ( ) ; return null ; }
evaluates the scd against the given context node and returns the matched node .
