the part argument to "date"

I don't quite like the part argument to "date", since the possible
values are listed explicitly.  making each of them a tagged argument is
easier to express in the grammar, and it avoids the problem of
"${format}" which can fail during run-time.  outline of suggested
change:


4.  Date Test

   Usage:   date [":zone" <time-zone: string>] [COMPARATOR]
                 [MATCH-TYPE] [PART] <header-name: string>
                 <key-list: string-list>
[...]

4.2.  Part argument

   The "PART" syntax element is defined as follows:

   Syntax:   ":year" / ":month" / ":day" [...]

   The optional part argument specifies a particular part of the
   resulting date/time value to match against the key-list.  If no part
   argument is given, ":iso8601" is used.  The available parts are:

   [description of each part suitably edited]

Ned, I can write a more specific patch if you like, if you send me your
source.

(BTW, I changed "parameter" to "argument" since that's what the base
spec calls them.)
-- 
Kjetil T.