actor model apache cocoon backtracking comefrom call-with-current-continuation call stack carl hewitt christopher strachey common lisp computing continuation-passing style control flow coroutine denotational semantics drew mcdermott dynamic memory allocation esoteric programming language exception handling function functional programming gnu goto inversion of control lambda calculus montague grammar parrot virtual machine perl prolog rife setcontext smalltalk spaghetti stack stackless python standard ml stateless server steve russell tail recursion unix system v unlambda web application framework web server