Mixins are functions that can be used to modularly extend Actor and Pawns. The "mix" and "with" operators are semantic sugar to make the construction of the composite class look nice. page 
Mixins are coming from Flavors. Flavors is an early Object System for the Lisp Machine.
Ward via matrix – When objects were new to me I dug deeply into both Favors (mixin lisp) and Smalltalk. Although I found the same delightful idiomatic application code in both, I found flavor's objects a jumble of variables while single-inheritance smalltalk object state was short and sweet.
I also studied The Art of the Interpreter which seemed much more careful about piling things into their mechanisms. A well formed scheme program would be the product of intention and care. A multi-xyz-inheritance solution is the sign of making some programmer's first thoughts expressible.