<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def find_and_modify(self, query=None, update=None):
 """ Finds documents in this collection that match a given query and updates them """

        update = update or {}

        for document in self.find(query=query):
            document.update(update)
            self.update(document)