readline(): RFC 2046, section 5.1.2 (and partially 5.1) both state that the

parser must recognize outer boundaries in inner parts.  So cruise through the
EOF stack backwards testing each predicate against the current line.

There's still some discussion about whether this is (always) the best thing to
do.  Anthony would rather parse these messages as if the outer boundaries were
ignored.  I think that's counter to the RFC, but might be practically more
useful.  Can you say behavior flag?  (ug).