* is true.
   *
   * <p>For example:
   *
   * {@snippet :
   * Equivalence<Person> SAME_AGE = Equivalence.equals().onResultOf(GET_PERSON_AGE);
   * }
   *
   * <p>{@code function} will never be invoked with a null value.
   *
   * <p>Note that {@code function} must be consistent according to {@code this} equivalence