Anyone who looks into my experimental outliner with notes, clown, is sure to notice that the native data format is EDN (Extensible Data Notation). The canonical data representation for outlines is OPML (Outline Processor Markup Language) and it is well suited for that purpose. It’s a dialect of XML especially adapted to outliners. So, why use EDN? Rich Hickey gives a good rationale at the link above. But for me it boils down to a few things.