### Question:
Empty extends ERE { @Override public ERE derive(Symbol s) { return empty; } private  Empty(); static Empty get(); @Override EREType getEREType(); @Override boolean equals(Object o); @Override int compareTo(Object o); @Override ERE copy(); @Override String toString(); @Override boolean containsEpsilon(); @Override ERE derive(Symbol s);  }### Answer:
@Test public void testDerive() { ERE derived = empty.derive(Symbol.get("test")); assertEquals(empty, derived); }