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abstract class IntList {
    /** Computes a String representation of this list wiht matching parentheses
      * as in Scheme.  For example, the list containing 1, 2 and 3 should return
      * (1, 2, 3) and the empty list should return ().
      * @return a non empty String consisting of elements in this list enclosed
      * in a pair of matching parenthesis, separated by commas.
      */
    abstract String listString();

    /** Accumulator helper method for listString to compute the String
      * required representation of this list given the accumulated
      * String representation of the preceding list.
      * @param acc the accumulated String representation of the list that
      * precedes this list.
      * @return a non empty String consisting of elements in this list enclosed
      * in a pair of matching parenthesis, separated by commas.
      */
    abstract String listStringHelp(String acc);
}


class EmptyIntList extends IntList {

    /** @return "()"*/
    String listString() { return "()"; }
    /** @param acc the accumulated String representation of the list that
      * precedes this list.  For example "(5, 3"
      * @return a non empty String consisting of elements in this list enclosed
      * in a pair of matching parenthesis, separated by commas.  For example,
      * "(5, 3)"
      */
    String listStringHelp(String acc) {
        return acc + ")";
    }
}

class ConsIntList extends IntList {
    int first;
    IntList rest;

    /** Calls on rest to perform the helper method listStringHelp passing it
      *   the accumulated String representation so far, which is "(" + first.
      * @return a non empty String consisting of elements in this list enclosed
      *    in a pair of matching parenthesis, separated by commas.
      */
    String listString() {
        return rest.listStringHelp("(" + first);
    }

    /** @param acc the accumulated String representation of the list that
      *   precedes this.  For example "(5, 3"
      * @return a non empty String consisting of elements in this list enclosed
      *   in a pair of matching parenthesis, separated by commas.  For example,
      *   "(5, 3)"
      */
    String listStringHelp(String acc) {
        return rest.listStringHelp(acc + ", " + first);
    }
}

/** Testing empty lists. */
class TestEmptyIntList extends TestCase {

    void test_listString() {
        EmptyIntList mt = new EmptyIntList();
        assertEquals(mt + ".listString()", "()", mt.listString());
    }
}