anchor.diff:      field name.
     VALUE-KIND: Category of the value type: :scalar, :message, :enum, etc.
     INDEX: Message field number of this map type.
  -  VAL-DEFAULT: Specified type for the default value of the maps value type."
  +  VAL-DEFAULT: Default value for the map entries, or nil to use $empty-default."
     (assert json-name)
     (assert value-kind)
     (check-type index integer)
  
positive.code: 
  (defmacro define-map (field-name &key key-type value-type json-name index
                                   value-kind val-default)
    "Define a lisp type given the data for a protobuf map type.
  
   Parameters:
    FIELD-NAME: Lisp name of the field containing this map.
  
negative.code: 
  DEFINE-EXTENSION:
  
  Creates an EXTENSION-DESCRIPTOR and stores it in the containing message. This
  descriptor simply defines the allowed range of indices for extending the
  message.
  
  DEFINE-EXTEND:
  
  