def METHOD_NAME(module, klass):
	'''Look for a subclass of klass in the module
	This function is used in several places in zim to get extension
	classes. Typically L{get_module()} is used first to get the module
	object, then this lookup function is used to locate a class that
	derives of a base class (e.g. PluginClass).
	@param module: module object
	@param klass: base class
	@note: don't actually use this method to get plugin classes, see
	L{PluginManager.get_plugin_class()} instead.
	'''
	subclasses = lookup_subclasses(module, klass)
	if len(subclasses) > 1:
		raise AssertionError('BUG: Multiple subclasses found of type: %s' % klass)
	elif subclasses:
		return subclasses[0]
	else:
		return None