You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: 04_selector.rb
    path of file: ./repos/jsonpath-rs/train
    
    the code of the file until where you have to start completion: require "pry"
require "pp"

class BaseNode
  attr_reader :id

  def to_s
    @id
  end
end

class NodeStr < BaseNode
  def initialize(id)
    raise "Id must be a string" unless id.is_a?(String)
    @id = id
  end
end

class NodeArr < BaseNode
  attr_reader :children

  def initialize(id, children)
    raise "Id must be a string" unless id.is_a?(String)
    raise "Children of NodeArr must be a Array" unless children.is_a?(Array)
    @id = id
    @children
    
  def next
    while @current do
      log
      log "Current: #{@current}"
      log "Stack size: #{@stack.size}"
      criterion = @criteria[@ci]

      if criterion
        if match?(@current.path_step, criterion)
          log "MATCH  #{@current.path_step} -> #{criterion}"

          if @criteria[@ci + 1].nil?
            item = @current.item
            @ci -= 1
            @current = @stack.pop
            log "RETURN ITEM"
            return item
          else
            @ci += 1
            @stack.push(@current)
            @current = @current.next
            unless @current
              @ci -= 1
              @stack.pop
              @ci -= 1
              @current = @stack.pop
            end
          end