question about partial body lengths

[sen_ml@xxxxxxxxxxx]

hello-

  i have some questions about the use of and processing of partial body 
lengths that i was hoping to get answers to :-)

  section 4.2.2.1 says:

    Each Partial Body Length header is followed by a portion of the
    packet body data. The Partial Body Length header specifies this
    portion's length. Another length header (of one of the three types --
    one octet, two-octet, or partial) follows that portion. The last
    length header in the packet MUST NOT be a partial Body Length header.
    Partial Body Length headers may only be used for the non-final parts
    of the packet.

  section 4.2.3 says:

    ... This is just one possible encoding, and many
    variations are possible on the size of the Partial Body Length
    headers, as long as a regular Body Length header encodes the last
    portion of the data. Note also that the last Body Length header can
    be a zero-length header.

  while processing an openpgp packet w/ partial body length headers,
once a partial body length is encountered, is it safe to assume that
the subsequent appearance of a non-'partial body length' header allows the
computation of the end of a packet?

  if not, how does one tell where the packet ends?  (what am i missing?)

  if yes, i may have missed something, but i don't see anything in the
specification that says that this must be the case.  (what am i missing?)

  thanks for your attention.

p.s. i checked the archives earlier today but didn't find any mention
of this.

p.p.s. on a separate (purely historical) note, for signature
subpackets, were there specific reasons why the subpacket length was
defined to come before the subpacket type and not the other way
around?