What do you think about this layout? It's pretty useless for big grammars or grammars with more complex layout of course, but it might be good for some small grammars like this. One immediately sees the hierarchical layout. I'm not sure I like the layout myself.

gistfile1.pl

grammar Rules {
	token TOP                                       { <line>*                                                }
	token    line                                   { <empty-line> | <field-specification-line> | <tag-line> }
	token        empty-line                         { [ '#' \V* ]? \v                                        }
	token        field-specification-line           { '!' <field-name> \v                                    }
	token                                field-name { 'to' | 'from' | 'subject'                              }
	token        tag-line                           { <tag-command> \h+ <conjunction>+ % [\h* '|' \h*] \v    }
	token                tag-command                { <sign> <tag-name>                                      }
	token                           sign            { ['+' | '-']                                            }
	token                           tag-name        { <alnum>+                                               }
	token                conjunction                { <condition_atom>+ % [\h+]                              }
	token                           condition_atom  { <+alnum +[.@-]>+                                       }
}