code: @Inline(value="$2.$3concat($1)", imported=Iterators.class)
  	public static <T> Iterator<T> flatten(Iterator<? extends Iterator<? extends T>> inputs) {
  		return Iterators.concat(inputs);
  	}
docs: Combines multiple iterators into a single iterator. The returned iterator traverses the
  elements of each iterator in {@code inputs}. The input iterators are not polled until necessary.
  
  @param inputs
  the to be flattened iterators. May not be <code>null</code>.
  @return an iterator that provides the concatenated values of the input elements. Never <code>null</code>.
  
  @since 2.13