code: public final <R> Ix<R> concatMap(IxFunction<? super T, ? extends Iterable<? extends R>> mapper) {
          return new IxFlattenIterable<T, R>(this, nullCheck(mapper, "mapper is null"));
      }
docs: Maps each element from this sequence into subsequent Iterable sequences whose elements are
  concatenated in order.
  <p>
  Note that flatMap and concatMap operations are the same in the Iterable world.
  <p>
  The result's iterator() forwards the call remove() to the current inner Iterator.
  
  @param <R> the result value type
  @param mapper the function
  @return the new Ix instance
  @throws NullPointerException if mapper is null
  @since 1.0
  @see #flatMap(IxFunction)