### Question:
ImmutableGroup implements CommandGroup { @Nonnull public static CommandGroup merge(@Nonnull CommandGroup... rootGroups) { return merge(Arrays.asList(rootGroups)); } private  ImmutableGroup(
      ImmutableGroup parent, Set<String> names, Set<? extends CommandCallable> commands); @Nonnull static CommandGroup merge(@Nonnull CommandGroup... rootGroups); @Nonnull static CommandGroup merge(@Nonnull Collection<? extends CommandGroup> rootGroups); @CheckReturnValue @Nonnull static ImmutableGroupBuilder builder(); @Nonnull static ImmutableGroup childlessRoot(@Nonnull Set<CommandCallable> commands); @Nonnull static ImmutableGroup copyOf(@Nonnull CommandGroup group); @Nonnull static ImmutableGroup of(@Nonnull T root, @Nonnull GroupAssembler<T> assembler); @Nonnull @Override Optional<ImmutableGroup> getParent(); @Nonnull @Override Set<ImmutableGroup> getChildren(); @Nonnull @Override Set<String> getNames(); @Nonnull @Override Set<? extends CommandCallable> getCommands(); @Override String toString(); @Override boolean equals(Object o); @Override int hashCode(); static final ImmutableGroup EMPTY; }### Answer:
@Test void testMerge() { CommandCallable first = AssembledCommandCallable.builder() .withLabels("first") .withAction((a, c) -> {}) .build(); CommandCallable second = AssembledCommandCallable.builder() .withLabels("second") .withAction((a, c) -> {}) .build(); CommandCallable third = AssembledCommandCallable.builder() .withLabels("third") .withAction((a, c) -> {}) .build(); CommandGroup one = ImmutableGroup.builder() .group("foo") .add(first) .group("bar") .parent() .parent() .build(); CommandGroup two = ImmutableGroup.builder() .group("foo") .add(second) .parent() .group("baz") .parent() .add(third) .build(); CommandGroup expected = ImmutableGroup.builder() .group("foo") .add(first) .add(second) .group("bar") .parent() .parent() .group("baz") .parent() .add(third) .build(); CommandGroup actual = ImmutableGroup.merge(one, two); assertEquals(expected, actual, "Groups were not merged correctly"); }