We decided to start with a small Lisp interpreter and then graft on exactly two more constructs: a way to make actors and a way to send messages.
Gerry had been studying and teaching Algol 60, so we decided to use the full funarg solution so that our toy language would have lexical scope.
Our intuition was that this would also keep track of actor’s acquaintances correctly.
(Also inspired by Algol 60, the first toy interpreter was call-by-name rather than call-by-value! I will gloss over that distinction here.)