+ job "only one"
+ pre (Whentools.one ())
+ every 10 seconds :
+ <<
+ # Takes longer than 10 seconds to run, but 'Whentools.one ()'
+ # will ensure only one is ever running.
+ sleep 11
+ >>
+
+When using pre functions, jobs must be given an explicit name, ie.
+you must use the C<job> statement.
+
+A number of pre functions are available in the library; see below.
+
+You can also write your own post functions (in OCaml). The function
+is passed one argument which is a C<Whentools.preinfo> struct, defined
+below. It should return a boolean: C<true> if the job should run, and
+C<false> if the job should not run.
+
+Note that a fresh serial number (see L</JOBSERIAL>) is assigned to
+each run, whether or not the job actually runs because of
+preconditions.
+
+=head3 POST FUNCTIONS
+
+After a job runs, you can control what happens to its output by
+writing a C<post> function. To write a post function you have to
+name the job (ie. have an explicit C<job> statement). Put C<post ...>
+after the job name like this:
+
+ job "poll source"
+ post (Whentools.mailto "you@example.com")
+ every 10 seconds :
+ <<
+ # ...
+ >>
+
+A number of post functions are available in the library; see below.
+
+You can also write your own post functions (in OCaml). The
+function is passed one argument which is a C<Whentools.result> struct,
+defined below.
+
+=head3 WHENTOOLS LIBRARY
+
+=head4 Functions
+
+=over 4
+
+=item B<Whentools.mailto> [I<~only_on_failure:true>]
+[I<~from:from_address>] I<email_address> I<result>
+
+This built-in post function sends the result of the script by email to
+the given email address.
+
+If the optional C<~only_on_failure:true> flag is set, then it is only
+sent out if the script failed.
+
+If the optional C<~from> flag is set, then the from address is set
+accordingly. This is sometimes needed when sending mail.
+
+Note the C<result> parameter is passed implicitly by the daemon. You
+do not need to add it.
+
+Here are some examples of using the mailto function:
+
+ job "ex.1"
+ post (Whentools.mailto "you@example.com")
+ every 10 seconds :
+ <<
+ # do something
+ >>