Duck is the command language for the Kumo proof assistant. Among its most important commands are proof rules, which transform proof tasks into subtasks. Additional commands tell where the source files for documentation are located, and provide titles and subtitles for the tatami pages.
For each Kumo website, the Duck code that generated it is linked to its homepage, so you can find many examples by going to the Kumo demos homepage, choosing an example, and then clicking on the duck icon to see the Duck code that generated that website.
Here is the syntax of Duck; a postscript version is also available.