class ActionDispatch::Journey::GTG::Builder

def build_followpos
  table = Hash.new { |h, k| h[k] = [] }.compare_by_identity
  @ast.each do |n|
    case n
    when Nodes::Cat
      lastpos(n.left).each do |i|
        table[i] += firstpos(n.right)
      end
    end
  end
  table
end