anchor.diff:  ;;; File: language.lisp
   ;;; Description: Code that implements the LISA programming language.
   ;;;
  -;;; $Id: language.lisp,v 1.20 2002/11/21 14:53:22 youngde Exp $
  +;;; $Id: language.lisp,v 1.21 2002/11/21 16:33:29 youngde Exp $
   
   (in-package "LISA")
   
  
positive.code: (defmacro defimport (class-name)
    `(import-class-specification ',class-name))
  
  (defmacro defcontext (context-name)
    `(unless (find-context (inference-engine) ,context-name nil)
       (register-new-context (inference-engine) 
                             (make-context ,context-name))))
  
  (defmacro undefcontext (context-name)
    `(forget-context (inference-engine) ,context-name))
  
negative.code:   (warn "RETRACT-INSTANCE is deprecated. Use RETRACT instead.")
    (parse-and-retract-instance instance (inference-engine)))
  
  (defun facts ()
    (let ((facts (get-fact-list (inference-engine))))
      (dolist (fact facts)
  