Many of those involved in Gregorian chant are probably familiar with the gabc notation format used by Gregorio, which is widely used for square notation typesetting, with a vast amount of freely available transcriptions.
Utilities converting Gregorio scores to LilyPond code have been available for years, but it's definitely more convenient to include gabc scores directly, without invoking an external utility - either pasting gabc code right in the LilyPond document, or referencing a gabc file by path.
It's still in an early stage of development, but fun things are already possible and support for more is planned.
Jakub