/** after matching the sequence, place the result of the <tt>idx</tt>th match in the output tree */
public static Sequence singleton(Element[] e, int idx) { return new Singleton(e, idx); }
/** after matching the sequence, place the result of the <tt>idx</tt>th match in the output tree */
public static Sequence singleton(Element[] e, int idx) { return new Singleton(e, idx); }