anchor.diff:       (destructuring-bind (operator &rest argument-forms) form
   
          ;; Sanity check -- this should happen at "compile time"
  -       (unless (symbolp operator)
  -         (error "~A is not a valid operator name" (car form)))
  +       (unless (or (symbolp operator) (listp operator))
  +         (error "~A is not a valid operator." (car form)))
   
          ;; Special operators
          (case operator
  
positive.code:             (unless (find-binding name fenv)
                (error "~A is not a lexically visible function" name))
              (lookup name fenv)))
           (|bind-lexical-functions|
            (destructuring-bind (vars-and-funs &body body)
                argument-forms
  
negative.code:           (destructuring-bind ((&rest args) &body body)
                argument-forms
              (make-function args body env fenv)))
           (|fun|
            (destructuring-bind (name)
                argument-forms
              ;; Sanity checks -- these should happen at "compile time"
  