code: public final Ix<T> distinctUntilChanged(IxPredicate2<? super T, ? super T> comparer) {
          return new IxDistinctUntilChanged<T, T>(this, IdentityHelper.<T>instance(), nullCheck(comparer, "comparer is null"));
      }
docs: Emits elements from this sequence if each element is different from the previous element
  (according to a comparer), dropping elements that evaluate to the same as the previous.
  <p>
  The result's iterator() doesn't support remove().
  @param comparer the predicate receiving the previous element and the current element and
  returns true if they are the same (thus ignoring the latter).
  @return the new Ix instance
  @throws NullPointerException if comparer is null
  @since 1.0