anchor.diff:       (cond
          ((and (not data-attribute)
                (not (attribute-optional-p schema-attribute)))
  -        (let ((error-msg (or (attribute-option :not-provided-message schema-attribute)
  -                             (format nil "Attribute ~a not provided"
  +        (let ((error-msg (or (attribute-option :attribute-required-message schema-attribute)
  +                             (format nil "Attribute required: ~a"
                                        (attribute-name schema-attribute)))))
             (validation-error error-msg)))
          ((not data-attribute)
  
positive.code:          (or (typep data 'local-time:timestamp)
               (and (stringp data)
                    (chronicity:parse data))))
      (validation-error "~A: ~A is not a timestamp"
                        (attribute-name attribute)
                        data)))
  
  
negative.code:          (or (typep data 'local-time:timestamp)
               (and (stringp data)
                    (chronicity:parse data))))
      (validation-error "~A: ~A is not a valid date"
                        (attribute-name attribute)
                        data)))
  
  