anchor.diff:  (def-expander-macro *tagbody-expander* tagbody (&body body)
     (tagbodyexpand body))
   
  -(define-compiler-macro tagbody (&body body)
  +(def-compiler-macro tagbody (&body body)
     (tagbodyexpand body))
   
   
  
positive.code: (def-expander-macro *block-expander* block (name &body body)
    (blockexpand name body))
  
  (define-compiler-macro block (name &body body)
    (blockexpand name body))
  
negative.code: 
  (var *compiler-macro-expander* (define-expander 'compiler :pre #'init-compiler-macros))
  
  (defmacro def-compiler-macro (name args &body x)
    (print-definition `(def-compiler-macro ,name ,args))
    `(def-expander-macro *compiler-macro-expander* ,name ,args ,@x))
  
  (fn compiler-macroexpand (x)
    (expander-expand *compiler-macro-expander* x))
  
  (def-compiler-macro cond (&rest args)
    (with-compiler-tag end-tag
      `(%%block
         ,@(mapcan [with-compiler-tag next
  