sub foo_a {                                                  
        my ($self, $arg) = @_;                                   
        # method A for processing $arg                           
    }                                                            
                                                                 
    sub foo_b {                                                  
        my ($self, $arg) = @_;                                   
        # method B for processing $arg                           
    }                                                            
                                                                 
    sub foo {                                                    
        my ($self, $arg) = @_;                                   
        $self->{'foo_code'}->($self, $arg);     # XXX            
    }                                                            
                                                                 
    sub foo_set {                                                
        my ($self, $code) = @_;                                  
        $self->{'foo_code'} = $code;                             
    }