Message passing of Smalltalk-72 was quite complex [Kay 1975]. Code in the language was viewed by the interpreter as simply a stream of tokens. According to [Ing

Message passing of Smalltalk-72 was quite complex [Kay 1975]. Code in the language was viewed by the interpreter as simply a stream of tokens. According to [Ingalls 1983]: "The first (token) encountered (in a program) was looked up in the dynamic context, to determine the receiver of the subsequent message. The name lookup began with the class dictionary of the current activation. Failing there, it moved to the sender of that activation and so on up the sender chain. When a binding was finally found for the token, its value became the receiver of a new message, and the interpreter activated the code for that object's class." Thus the message passing model in Smalltalk-72 was closely tied to a particular machine model and programming language syntax that did not lend themselves to concurrency. SENDER was retained as part of the message-passing protocol, which is problematical for distributed systems. Also, although the system was bootstrapped on itself, the behavior of language constructs was defined (like Lisp) by an interpreter instead by their response to eval messages.