This example shows how to use @ytt:library module to programmatically template custom library app (found _ytt_lib/app). See comments in config.yml for more details:
# turn _ytt_lib1 -> _ytt_lib
$ mv ./examples/playground/example-ytt-library-module/_ytt_lib{1,}
$ ytt -f ./examples/playground/example-ytt-library-module