App

The App class (Processing::App) is a subclass of PApplet, and one way or another your propane sketch inherits from Processing::App, it is just not obvious in a nude or bare propane sketch. Unless you explicitly create your own Sketch class, the default is to create an instance of Sketch < Processing::App as below. What a bare sketch would look like when wrapped prior to eval.

class Sketch < Processing::App
  # your bare sketch code may include 'draw' should include 'settings'
end

Sketch.new

What a nude static sketch would look like when wrapped prior to eval.

class Sketch < Processing::App
  def setup
    sketch_title 'Nude Sketch'
    # your nude sketch code here eg `background 0`
    no_loop
  end
  def settings
    size(150, 150)
  end
end

Sketch.new

You should be aware that all vanilla processing sketches are similary wrapped prior to java compilation. The idea of processing and also propane is to make it easy to create sketches, so this is all you need to know, and you should probably look at propane if you really prefer class wrapped sketches.

For propane see also Processing module for more details, you could also see magic (but it ‘really’ is not necessary)