return runnable.toString();
    }
  }

  private interface Adder {
    int add(int a, int b);
  }

  private static class ForwardingArithmetic implements Arithmetic {
    private final Arithmetic arithmetic;

    public ForwardingArithmetic(Arithmetic arithmetic) {
      this.arithmetic = arithmetic;
    }

    @Override
    public int add(int a, int b) {
      return arithmetic.add(a, b);