Return the list of descendants of a class

descendants.rb

class Class
  def extend?(klass)
    superclass && (superclass == klass || superclass.extend?(klass))
  end

  def descendants
    Module.constants.select do |constant_name|
      constant = eval(constant_name.to_s)
      if constant && constant.is_a?(Class) && constant.extend?(self)
        constant
      end
    end
  end
end

# ruby-1.9.2-p290 :002 > StandardError.descendants
# => [:TypeError, :ArgumentError, :IndexError, :KeyError, :RangeError, :NameError, :NoMethodError, :RuntimeError, :EncodingError, :SystemCallError, :ZeroDivisionError, :FloatDomainError, :RegexpError, :IOError, :EOFError, :LocalJumpError, :StopIteration, :ThreadError, :FiberError]