nullable?

# File actionpack/lib/action_dispatch/journey/gtg/builder.rb, line 63
        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