“[Searching for] more and more powerful ways to express and manipulate control structure to support heuristic search in MIT’s quest for the ‘ultimate AI language’, we had come full circle from Lisp to CONVERT to Planner to Conniver to PLASMA to a simple variation of Lisp!

The original theoretical core of Lisp was recursion equations, not the lambda-calculus. An important consequence of the existence of a [scheme] interpreter for the lambda-calculus was that suddenly a denotational semantics could be regarded as an operational semantics.