anchor.diff:   amatch arg &body clauses
    Same as MATCH, except that handling of algebraic types is enabled"
     (if (not (boundp 'abacus-typespec))
  -    (defvar abacus-typespec nil))
  +      (defvar abacus-typespec nil))
     `(let  ((abacus-it nil))
        (match ,arg ,@clauses)))
   
  
positive.code: 
  (defvar *readtables-stack* nil)
  
  (defconstant left-bracket #\[)
  (defconstant right-bracket #\])
  
  (defmacro enable-match-syntax ()
  
negative.code:            #:algebraic-match-with
             #:algebraic-guard
             #:enable-match-syntax
             #:disable-match-syntax
             :left-bracket
             :right-bracket
             :*readtables-stack*))
  