### Question:
Symbol extends ERE { @Override public ERE derive(Symbol s) { if(this == s) { return Epsilon.get(); } return Empty.get(); } private  Symbol(); static Symbol get(String name); @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() { Empty empty = Empty.get(); Epsilon epsilon = Epsilon.get(); assertEquals(epsilon, one.derive(one)); assertEquals(empty, one.derive(two)); assertEquals(epsilon, two.derive(two)); assertEquals(empty, two.derive(one)); }