Remove (next-week) from texinfo documentation, discuss equivalent as example.

It has been decided not to implement the (next-week) procedure in the program,
so we need to remove the references to it in the manual.

The reason for the omission is discussed in a new extended example section,
and code is provided to illustrate how the functionality can be implemented if
needed.

The change also makes some alteration to the flow of sections in the document,
and implicitly fixes some problems in the flow that the previous document had,
with regards to linking the GNU Free Documentation License in the document
structure.

   * doc/mcron.texi:  modifications as described.

doc/mcron.texi

@@ -85,6 +85,7 @@ Full available syntax
 Extended Guile examples
 
 * AT commands::
+* Weekly::
 * Every second Sunday::
 * Two hours every day::
 * Missing the first appointment::
@@ -309,12 +310,11 @@ of jobs that mcron has to start at the same time.}
 @findex next-minute-from
 @findex next-hour-from
 @findex next-day-from
-@findex next-week-from
 @findex next-month-from
 @findex next-year-from
 Similarly to @code{next-second-from}, there are also
 @code{next-minute-from}, @code{next-hour-from}, @code{next-day-from},
-@code{next-week-from}, @code{next-month-from}, @code{next-year-from}.
+@code{next-month-from}, @code{next-year-from}.
 
 @findex iota
 Furthermore, the optional argument can be fulfilled by the Guile
@@ -333,7 +333,6 @@ example @code{(range 0 10 2)} will also yield the list @code{'(0 2 4 6
 @findex next-minute
 @findex next-hour
 @findex next-day
-@findex next-week
 @findex next-month
 @findex next-year
 @cindex time specification, list
@@ -413,13 +412,14 @@ demonstrate how to code around them.
 
 @menu
 * AT commands::
+* Weekly::
 * Every second Sunday::
 * Two hours every day::
 * Missing the first appointment::
 * Penultimate day of every month::
 @end menu
 
-@node AT commands, Every second Sunday, Extended Guile examples, Extended Guile examples
+@node AT commands, Weekly, Extended Guile examples, Extended Guile examples
 @subsection Synthesizing ``at'' commands
 @cindex at command
 The current implementation of mcron does not provide for an @emph{at}
@@ -438,7 +438,27 @@ superfluous entries).
                 (kill (getppid) SIGINT)))
 @end example
 
-@node Every second Sunday, Two hours every day, AT commands, Extended Guile examples
+@node Weekly, Every second Sunday, AT commands, Extended Guile examples
+@subsection Weekly
+@cindex examples, weekly
+The astute reader will have noticed that there are no @code{next-week} or
+@code{next-week-from} functions.  This is because these concepts are fraught
+with ambiguity: does ``next week'' mean seven days from now, or next Sunday,
+or next Monday, or what?  If the month does not start on a week boundary, are
+the first few days considered the first week, or does the first week begin on
+the first Sunday (or Monday)?  This is important because, for example, trying
+to specify the second Thursday in a month might actually get the third one if
+the latter interpretation is held.  Because of this we do not provide these
+functions.
+
+If you really want a job to run every seven days, you could use code like
+
+@example
+(job (lambda (current-time) (+ current-time (* 7 24 60 60)))
+     "my-program")
+@end example
+
+@node Every second Sunday, Two hours every day, Weekly, Extended Guile examples
 @subsection Every second Sunday
 @cindex examples, every second sunday
 To run @code{my-program} on the second Sunday of every month, a Guile
@@ -1207,7 +1227,7 @@ Cron has been run by a user other than root.
 
 
 
-@node Guile modules, Index, Invoking, Top
+@node Guile modules, GNU Free Documentation License, Invoking, Top
 @chapter Guile modules
 Some of the key parts of mcron are implemented as modules so they can
 be incorporated into other Guile programs, or even into C-sourced
@@ -1438,12 +1458,12 @@ Once this module has been declared in a program, a crontab file can be
 used to augment the current job list with a call to
 @code{read-vixie-file}.
 
-@node GNU Free Documentation License
+@node GNU Free Documentation License, Index, Guile modules, Top
 @appendix GNU Free Documentation License
 
 @include fdl.texi
 
-@node Index,  , Guile modules, Top
+@node Index,  , GNU Free Documentation License, Top
 @unnumbered Index
 
 @printindex cp