Adding "From_" as an X-header

Neither of my mail clients display the "From_" envelope line, and I'm
always curious as to what it is. So I thought I would add a formail
filter at the top of my .procmailrc to pick it up and insert it into
each mail as an X-header, so I can see it in my mail client. Here is
my test recipe:

:0
* ^Subject.*formailtest
{
 ENVELOPE=`formail -xFrom `
 :0f
 | formail -I"X-Envelope: $ENVELOPE"
}

This recipe does not fail or give errors, but it seems to assign both
the "From_" *and* the "From:" lines to $ENVELOPE, with a newline
between them. Mail to myself looks like this:

[snip]
Content-Type: TEXT/PLAIN; charset=US-ASCII
X-Envelope: revjack(_at_)saltmine(_dot_)radix(_dot_)net  Sat May 31 22:09:36 
1997
 Jack Lyons <revjack(_at_)Radix(_dot_)Net>
Status: RO
X-Status:

Does anyone know how to tell formail to grab only the "From_" line?
_________________
revjack(_at_)radix(_dot_)net