<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def dict_to_trees(self, expansion_dict):
 """ Convert an expansion dictionary to a list of expansion trees. :param expansion_dict: An expansion dictionary (see below) :type expansion_dict: dict :return: a list of expansion trees (`TreeNode` instances). :rtype: list[TreeNode] Expansion Dictionary Format: { } <type> is the type of object to expand. <expansion string> is a string with the following format: <expansion string> => <expansion name>[.<expansion string>] """

        trees = []
        for node_type, expansion_list in six.iteritems(expansion_dict):
            type_node = TypeNode(node_type=node_type)

            for expansion_string in expansion_list:
                expansion_node = type_node

                for expansion_name in expansion_string.split('.'):
                    child_expansion_node = expansion_node.get_expansion(expansion_name)

                    if not child_expansion_node:
                        type_expansion = self.type_expansions[expansion_node.type][expansion_name]
                        type_route = self.type_routes[type_expansion['route']]
                        if type_expansion['destination_field'] == type_expansion['source_field']:
                            raise ValueError(
                                'Expansion configuration destination_field error: '
                                'destination_field can not have the same name as the source_field: '
                                '{}'.format(type_expansion['source_field'])
                            )
                        child_expansion_node = ExpansionNode(
                            node_type=type_expansion['type'],
                            name=expansion_name,
                            source_field=type_expansion['source_field'],
                            destination_field=type_expansion['destination_field'],
                            service=type_route['service'],
                            action=type_route['action'],
                            request_field=type_route['request_field'],
                            response_field=type_route['response_field'],
                            raise_action_errors=type_expansion.get('raise_action_errors', False),
                        )
                        expansion_node.add_expansion(child_expansion_node)

                    expansion_node = child_expansion_node

            trees.append(type_node)

        return trees