### Question:
Kleene extends ERE { @Override public ERE derive(Symbol s) { return Concat.get(children.get(0).derive(s), copy()); } private  Kleene(ERE child); static ERE get(ERE child); @Override EREType getEREType(); @Override String toString(); @Override ERE copy(); @Override boolean containsEpsilon(); @Override ERE derive(Symbol s);  }### Answer:
@Test public void testDerive() { Epsilon epsilon = Epsilon.get(); Empty empty = Empty.get(); assertEquals(aStar, aStar.derive(a)); assertEquals(empty, aStar.derive(b)); ERE ab = Concat.get(a, b); ERE ab_Star = Kleene.get(ab); assertEquals(ab_Star, ab_Star.derive(a).derive(b)); assertEquals(empty, ab_Star.derive(b)); }