role Node;

class Conjunction with Node {
   has left  => (is => 'ro', does => 'Node');
   has right => (is => 'ro', does => 'Node');
}

class Disjunction with Node {
   has left  => (is => 'ro', does => 'Node');
   has right => (is => 'ro', does => 'Node');
}

class Negation with Node {
   has negated_term => (is => 'ro', does => 'Node');
}

class Primative with Node {
   has label => (is => 'ro', isa => Str);
}

my $fred = Primative->new(label => "Fred");
my $john = Primative->new(label => "John");
my $fred_and_john = Conjunction->new(left => $fred, right => $john);
...;

##</code><code>##

my $demorgans_laws = sub
{
   my $tree = $_[0];
   
   if ($tree->isa('Conjunction')
   and $tree->left->isa('Negation')
   and $tree->right->isa('Negation'))
   {
      return Negation->new(
         node => Disjunction->new(
            left => $tree->left->negated_term,
            right => $tree->right->negated_term,
         ),
      );
   }
   
   if ($tree->isa('Disjunction')
   and $tree->left->isa('Negation')
   and $tree->right->isa('Negation'))
   {
      return Negation->new(
         node => Conjunction->new(
            left => $tree->left->negated_term,
            right => $tree->right->negated_term,
         ),
      );
   }
   
   return $tree;
};