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);
...;