This doesn't work because the block can't be passed recursively.

broken_dfs.rb

class TreeNode
   # ... omitted.
  
  def dfs(target_value = nil, &blk)
  # A (broken) first attempt at depth-first search
  # with arbitrary search criteria defined by the 
  # block received. 
    return self if !blk.nil && blk.call(@value)
    return self if @value == target_value
 
    found_node = nil 
    @children.each do |child|
      found_node = child.dfs(target_value, blk) # Argument error.
      return found_node unless found_node.nil?
    end
    
    found_node
  end
  
  # ...
end