anchor.diff:                (destructuring-bind (name)
                     argument-forms
                   ;; Sanity checks -- these should happen at "compile time"
  -                (unless (symbolp name)
  +                (unless (clutter-symbol-p name)
                     (error "~A is not a valid function name" name))
                   (unless (find-binding name fenv)
                     (error "~A is not a lexically visible function" name))
  
positive.code:               (destructuring-bind (variable value)
                    argument-forms
                  ;; Sanity checks -- these should happen at "compile time"
                  (unless (symbolp variable)
                    (error "~A is not a valid function name." variable))
                  (unless (find-binding variable env)
                    (error "~A is not a lexically visible function." variable))
  
negative.code:              ((eq operator (clutter-intern "define-global-function"))
                (destructuring-bind (name value)
                    argument-forms
                  (unless (clutter-symbol-p name)
                    (error "~A is not a valid variable name." name))
                  (let ((fn (evaluate value env fenv)))
                    (unless (clutter-function-p fn)
  