;; S : STD11 structured field
;; M : MIME structured field
;; U : Unstructured field
+;; N : Non-scanning field
lt ; S/M
gt ; S/M
us-wsp ; U
us-fold ; U
+n-body ; N
+
; Productions
(start (tag-mailbox gap mailbox) : ()
(tag-mailbox* gap mailbox*) : ()