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;
};