anchor.diff:  
   (defmacro null-pointer-p (ptr)
     "Return true if PTR is a null pointer."
  -  `(cffi:null-ptr-p ,ptr))
  -
  -(defparameter +null-cstring-pointer+ (cffi:null-ptr)
  +  `(cffi:null-pointer-p ,ptr))
  +
  +(defparameter +null-cstring-pointer+ (cffi:null-pointer)
     "A constant NULL string pointer.")
   
   (defmacro char-array-to-pointer (obj)
  
positive.code:   (let ((str (gensym)))
      `(let ((,str ,lisp-string))
         (if (null ,str)
             (let ((,foreign-string (cffi:null-ptr)))
               ,@body)
             (cffi:with-foreign-string (,foreign-string ,str)
               ,@body)))))
  
negative.code:   (let ((str (gensym)))
      `(let ((,str ,obj))
         (if (null ,str)
             (cffi:null-pointer)
             (cffi:foreign-string-alloc ,str)))))
  
  (defmacro free-cstring (ptr)
  