Yesterday I built most of the arm, and designed the sling holder. Today I started building the secondary and tertiary trigger assembly (they are timed separately). When discussing the design with my dad yesterday, I think we came up with a better way to hold the sling, and also a possible design modification. This thing is complex enough that I'm having trouble visualizing it working. Mathematically it may be the simplest treb, but mechanically it is quite the opposite. On a related note, I think I finally understand how to model arms and slings mathematically.
Treb Construction Progress