Add fork_exec example.

pep-0437.txt

@@ -261,6 +261,28 @@ formatted this way::
     /*[define_output_end]*/
 
 
+Benefits of a compact notation
+==============================
+
+The advantages of a concise notation are especially obvious when a large
+number of parameters is involved. The argument parsing part of
+``_posixsubprocess.fork_exec`` is fully specified by this definition::
+
+    /*[define subprocess_fork_exec]
+    def _posixsubprocess.fork_exec(
+        process_args: "O", executable_list: "O",
+        close_fds: "p", py_fds_to_keep: "O",
+        cwd_obj: "O", env_list: "O",
+        p2cread: "i", p2cwrite: "i", c2pread: "i", c2pwrite: "i",
+        errread: "i", errwrite: "i", errpipe_read: "i", errpipe_write: "i",
+        restore_signals: "i", call_setsid: "i", preexec_fn: "i", /) -> int: pass
+    [define_end]*/
+
+
+Note that the *preprocess* tool currently emits a redundant C-declaration
+section for this example, so the output is longer than necessary.
+
+
 Easy validation of the definition
 =================================