.PP
.B one message per file
.RS 5
-The file name is a positive integer. The filename for a new
+The file name is a positive integer. Other files containing metadata or
+arbitrary names can exist in a folder; while the preference is that non\-message
+files begin with \*(lq.\*(rq, all files that are not positive integers
+.B must
+be ignored by a
+.BR MH \-compatible
+implementation. However, implementations are free to indicate to the user
+the existence of non\-message files that are not prefixed with a \*(lq.\*(rq.
+.PP
+The filename for a new
message is one greater than the highest numbered message in
-the folder; its full path can be shown by
-.B mh-path
-.IR new .
+the folder; its full path can be accessed by the pseudo\-sequence
+.I new
+(e.g.,
+.B mhpath
+.IR new ).
+New messages are
+.B only
+permitted to be added to a folder at the end of the message number range.
+.PP
+To add a new message to a folder, the recommended sequence is:
+.PP
+.IP \(bu 4
+Create a temporary file in the desired folder.
+.IP \(bu 4
+Attempt to link the temporary file to the new message number.
+.IP \(bu 4
+If successful, remove the temporary file. If the link fails increment the
+message number and try again.
.PP
.RE
.B context
numbers in each sequence. The
.B cur
sequence has at most just a single message number, not a range.
+.PP
+Sequence names have a maximum size of 998 characters. Each line is also
+limited to a maximum of 998 characters, but RFC\-822 continuation rules
+apply; sequences can be continued across multiple lines by prefixing
+continuation lines with a whitespace character.
+.PP
+If an implementation finds messages in a sequence that do not exist,
+the sequence file should be updated to remove the missing messages
+from the sequence. If a sequence contains no messages, it should be
+removed from the sequence file. The exception to this is the
+.B cur
+sequence, which can refer to a nonexistant message.
.RE
.PP
.SS Locking