code: public <U extends T> OngoingMatchingC0<T, U> when(
        DecomposableMatchBuilder0<U> decomposableMatchBuilder) {
      return new OngoingMatchingC0<>(this, decomposableMatchBuilder.build());
    }
docs: Specifies an match on a decomposing matcher with 0 extracted fields and then returns a fluent
  interface for specifying the action to take if the value matches this case.