I'm tending to the opinion that the words 'non-empty' should simply be
deleted from section 2 of -pra. They seem to serve no purpose other
that to add complexity, and for all the header fields in question,
empty field bodies are illegal, and I don't believe they are prevelent
in the wild.
If they must remain, then the wording should be made more accurate, ie
"Locate the first non-empty Resent-Sender header in the message."
should become
"Locate the first Resend-Sender field in the message that has a
non-empty field body"
and similarly for the other references.