+=head3 PRE FUNCTIONS
+
+Before a job runs, you can arrange that a C<pre> function is called.
+This function may decide not to run the job (by returning C<false>).
+
+One use for this is to prevent a particular job from running if there
+is already an instance of the same job running:
+
+ 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.
+