code: public final @NotNull S hasSize(int expected) {
      isNotNull();
      int size = actualGroupSize();
      if (size == expected) {
        return myself();
      }
      failIfCustomMessageIsSet();
      throw failure(format("expected size:<%s> but was:<%s> for <%s>", expected, size, actual));
    }
docs: Verifies that the number of values in the actual group is equal to the given one.
  
  @param expected the expected number of values in the actual group.
  @return this assertion object.
  @throws AssertionError if the number of values of the actual group is not equal to the given one.