OK, enclosed is my proposed re-wording for z=.
eric
Copied header fields (plain-text, but see description; OPTIONAL,
default is null). A vertical-bar-separated list of selected header
fields present when the message was signed, including both the field
name and value. It is not required to include all header fields
present at the time of signing. This field need not contain the same
header fields listed in the "h=" tag. The syntax used resembles
Quoted-Printable [RFC2045]: any character MAY be encoded as an "="
followed by two hexadecimal digits from the alphabet
"0123456789ABCDEF" (no lower case characters permitted) representing
the internal value of that character. All control characters (those
with values < %x20), eight-bit characters (values > %x7F), and the
characters DEL (%x7F), SPACE (%x20), vertical bar ("|", %x7C), colon
(":", %x3A), and semicolon (";", %x3B) MUST be encoded in both the
header field name and the header field value. Note that all white
space, including CR and LF characters, must be encoded. After
encoding, SWSP MAY be added at arbitrary locations in order to avoid
excessively long lines; such white space is NOT part of the value of
the header field, and MUST be removed before decoding.
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