# File lib/journey/gtg/builder.rb, line 62
      def nullable? node
        case node
        when Nodes::Group
          true
        when Nodes::Star
          true
        when Nodes::Or
          node.children.any? { |c| nullable?(c) }
        when Nodes::Cat
          nullable?(node.left) && nullable?(node.right)
        when Nodes::Terminal
          !node.left
        when Nodes::Unary
          nullable? node.left
        else
          raise ArgumentError, 'unknown nullable: %s' % node.class.name
        end
      end