code: public MethodHandle guard(MethodHandle target, MethodHandle fallback) {
          return MethodHandles.guardWithTest(handle, target, fallback);
      }
docs: Use this SmartHandle as a test to guard target and fallback handles.
  
  @param target   the "true" path for this handle's test
  @param fallback the "false" path for this handle's test
  @return a MethodHandle that performs the test and branch