    <ipv4>      ::= NUMBER ('.') NUMBER ('.') NUMBER ('.') NUMBER  action => joindot
    <hostname>  ::= NAMECH+                      separator => DOT  action => joindot
    <hostaddr4> ::= <ipv4>                                         action => add_ip
                  | <hostname>                                     action => add_host
...
# There should be a semicolon, not a comma.
#                  |
#                  v
sub joindot { shift; join '.', @_ }
sub AUTOLOAD {
    die 'Invalid action' unless $::AUTOLOAD =~ /^add_(ip|host)$/;
    {type => $1, $1 => $_[1]}
}