sub new {
   my $self = bless {
     alpha => {
       value  => undef,
       set_ok => 1,
       get_ok => 1,
       set    => sub {
         # untested.
         my $self = shift;
         $self->_set_alpha(@_);
       },
       get    => sub {
         my $self = shift;
         $self->{alpha}{value};
       },
       valid  => sub {
         my $val = shift;
         return ($val =~ m/^\d+$/) ? 1 : 0; #only digits
       },
     },
   }, shift;

   return $self;
}   

sub attrib {
   my $self = shift;
   my ($attr, $value) = @_;
   
   die("No attribute '$attr'")
     unless (ref $self->{$attr} eq 'HASH');

   # set
   if (scalar @_ >= 2) {
      die("Not allowed to set '$attr'") unless $self->{$attr}{set_ok};
      die("Invalid value for '$attr'") 
        unless $self->{$attr}{valid}->($value);
      $self->{$attr}{set}->($value);
   }
   else {
      die("Not allowed to get '$attr'") unless $self->{$attr}{get_ok};
      return $self->{$attr}{get}->();
   }
}