concurrent_nested_hash.rb

require 'celluloid'

class ConcurrentNestedHash
  include Celluloid

  def initialize
    @outer = {}
  end

  def [](*keys)
    keys.inject(@outer) { |h,k| h[k] }
  end

  def []=(*args)
    value = args.pop
    raise ArgumentError, "wrong number of arguments (1 for 2)" if args.empty?

    key  = args.pop
    hash = args.inject(@outer) { |h,k| h[k] ||= {} }
    hash[key] = value
  end
end

Wait I got it. def sort_by( &block ); @outer.sort_by &block end

Missed the second ampersand inside the method, not just in the definition (when passing a block and not *args).