code: public <U extends T> OngoingMatchingC0<T, U> when(MatchesExact<U> o) {
      List<Matcher<Object>> matchers = Lists.of(ArgumentMatchers.eq(o.t));
      return new OngoingMatchingC0<>(
          this, new DecomposableMatchBuilder0<>(matchers, new IdentityFieldExtractor<U>()).build());
    }
docs: Specifies an exact match and then returns a fluent interface for specifying the action to take
  if the value matches this case.