code: public <B> ISeq<B> map(final Function<FlatTreeNode<T>, ? extends B> mapper) {
  		return stream()
  			.map(mapper)
  			.collect(ISeq.toISeq());
  	}
docs: Return a sequence of all <em>mapped</em> nodes of the whole underlying
  tree. This is a convenient method for
  <pre>{@code
  final ISeq<B> seq = stream()
  .map(mapper)
  .collect(ISeq.toISeq())
  }</pre>
  
  @param mapper the mapper function
  @param <B> the mapped type
  @return a sequence of all <em>mapped</em> nodes