The Smalltalk-72 model was extremely limited. Code in the language was viewed by the interpreter as simply a stream of tokens. According to [Ingalls 1983]:

The Smalltalk-72 model was extremely limited. 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." SENDER and stack of previous senders was retained as part of the message-passing protocol, which is problematical for concurrent systems.