t.pushRight(2).right == 2

    t.mapLeft { it + 1 }.left == 2
    t.mapLeft { it + 1 }.right == "a"
    t.mapRight { it * 2 }.left == 1
    t.mapRight { it * 2 }.right == "aa"

    t.nestLeft(2).mapLeft(unpackLeft()).left == 2
    t.nestLeft(2).mapLeft(unpackRight()).left == 1
    t.nestLeft(2).mapLeft(unpackLeft()).right == "a"
    t.nestLeft(2).mapLeft(unpackRight()).right == "a"

    /**
     * Map the left side.
     *
     * @see #mapRight
     * @see #fold
     */
    public abstract <U, V> Either<U, V> mapLeft(Function<? super L, ? extends U> f);

    /**
     * Map the right side.
     *
     * @see #mapLeft
     * @see #fold
     */
    public abstract <U, V> Either<U, V> mapRight(Function<? super R, ? extends V> f);

    /**

        right.right.get() == RIGHT
    }

    def "map() works"() {
        expect:
        left.mapLeft({ assert it == LEFT; LEFT2 }).left.get() == LEFT2
        left.mapRight({ assert false }).left.get() == LEFT

        right.mapRight({ assert it == RIGHT; RIGHT2 }).right.get() == RIGHT2
        right.mapLeft({ assert false }).right.get() == RIGHT
    }

    def "fold() subsumes flatMap()"() {
        expect:

        return of(of(t, left), right);
    }

    public <T> Pair<L, Pair<T, R>> nestRight(T t) {
        return of(left, of(t, right));
    }

    public <T> Pair<T, R> mapLeft(Function<? super L, ? extends T> function) throws Exception {
        return of(function.apply(left), right);
    }

    public <T> Pair<L, T> mapRight(Function<? super R, ? extends T> function) throws Exception {

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