code: public <U extends T, A> OngoingMatchingR1<T, U, A, R> when(
        DecomposableMatchBuilder1<U, A> decomposableMatchBuilder) {
      return new OngoingMatchingR1<>(this, decomposableMatchBuilder.build());
    }
docs: Specifies an match on a decomposing matcher with 1 extracted fields and then returns a fluent
  interface for specifying the action to take if the value matches this case.