# File lib/abstract_node.rb, line 12
  def initialize(data=nil, *sub_nodes)
    @data = data
    self.each_node { |sub_node| check_sub_node_type(sub_node) }
  end

# File lib/abstract_node.rb, line 19
  def [](index)
    @sub_nodes[index]
  end

# File lib/abstract_node.rb, line 23
  def []=(index, sub_node)
    check_sub_node_type(sub_node)
    @sub_nodes[index] = sub_node
  end

# File lib/abstract_node.rb, line 46
  def delete(index)
    @sub_nodes.delete(index)
  end

# File lib/abstract_node.rb, line 42
  def each_index(&block)
    @sub_nodes.each_pair { |index, sub_node| block[index] }
  end

# File lib/abstract_node.rb, line 36
  def each_node(&block)
    @sub_nodes.each_pair { |index, sub_node| block[sub_node] }
  end

# File lib/abstract_node.rb, line 32
  def each_pair(&block)
    @sub_nodes.each_pair { |index, sub_node| block[index, sub_node] }
  end

# File lib/abstract_node.rb, line 57
  def leaf?
    @sub_nodes.empty?
  end

# File lib/abstract_node.rb, line 28
  def merge!(sub_nodes)
    sub_nodes.each { |index, sub_node| self[index] = sub_node }
  end

# File lib/abstract_node.rb, line 50
  def nb_sub_nodes
    @sub_nodes.size
  end

# File lib/abstract_node.rb, line 61
  def pre_depth_first(index=nil, &block)
    block[index, self]
    @sub_nodes.each_pair do |index, sub_node|
      sub_node.pre_depth_first(index, &block)
    end
    nil
  end

# File lib/abstract_node.rb, line 74
  def check_sub_node_type(sub_node)
    unless sub_node.is_a?(self.class)
      raise(TypeError, "`#{sub_node}' - must be a #{self.class}")
    end
  end