code: public static <D, R> Iterator<R> apply(Mapping<? super D, R> mapping, Iterator<? extends D> baseIt) {
          return Iterators.transform(baseIt, mapping::get);
      }
docs: Applies a mapping to an iterator. For the behavior, see {@link #apply(Mapping, Iterable)}. The resulting iterator
  supports each operation which the underlying supports.
  
  @param <D>
  domain class.
  @param <R>
  range class.
  @param mapping
  the mapping to apply.
  @param baseIt
  the underlying iterator.
  
  @return the mapped iterator.