firstpos

# File actionpack/lib/action_dispatch/journey/gtg/builder.rb, line 84
        def firstpos(node)
          case node
          when Nodes::Star
            firstpos(node.left)
          when Nodes::Cat
            if nullable?(node.left)
              firstpos(node.left) | firstpos(node.right)
            else
              firstpos(node.left)
            end
          when Nodes::Or
            node.children.flat_map { |c| firstpos(c) }.uniq
          when Nodes::Unary
            firstpos(node.left)
          when Nodes::Terminal
            nullable?(node) ? [] : [node]
          else
            raise ArgumentError, "unknown firstpos: %s" % node.class.name
          end
        end