(defconstant no-bindings '((t . t)))

(defun get-binding (var bindings)
  "Find a (var . value) pair in a binding list"
  (assoc var bindings))

(defun binding-val (bindings)
  "Get the value part of a single binding"
  (cdr binding))

(defun lookup (var bindings)
  "Get the value part (for var) for a binding list"
  (binding-val (get-binding var bindings))

(defun extend-bidings 
  "Add a (var . value) pair to a binding list"
  (cons (cons var val)
        (if (eq bindings no-bindings)
            nil
            bindings)))