doc sub USAGE

Author: gfldex

doc/Language/functions.pod6

@@ -808,7 +808,6 @@ Coercion types are supposed to work wherever types work, but Rakudo
 currently (2015.02) only implements them for subroutine parameters.
 
 =head1 C<sub MAIN>
-
 X<|MAIN>X«|command line arguments»
 
 The sub with the special name C<MAIN> is executed after all relevant phasers,
@@ -833,6 +832,23 @@ call L<exit|https://docs.perl6.org/routine/exit>.
         exit 1;
     }
 
+=head1 C<sub USAGE>
+X<|USAGE>
+
+If the no multi candidate of C<MAIN> is found for the given command line
+parameters, the sub C<USAGE> is called. If no such method is found, output a
+generated usage message.
+
+    sub MAIN(Int $i){ say $i == 42 ?? 'answer' !! 'dunno' }
+
+    sub USAGE(){
+    print Q:c:to/EOH/;
+    Usage: {$*PROGRAM-NAME} [number]
+
+    Prints the answer or 'dunno'.
+    EOH
+    }
+
 =end pod
 
 # vim: expandtab shiftwidth=4 ft=perl6