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Solve the following problem using Python, implementing the functions described below, one line at a time
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<USER_TASK:>
Description:
 def search_assignable_users_for_projects(self, username, projectKeys, startAt=0, maxResults=50):
 """Get a list of user Resources that match the search string and can be assigned issues for projects. :param username: A string to match usernames against :type username: str :param projectKeys: Comma-separated list of project keys to check for issue assignment permissions :type projectKeys: str :param startAt: Index of the first user to return (Default: 0) :type startAt: int :param maxResults: Maximum number of users to return. If maxResults evaluates as False, it will try to get all users in batches. (Default: 50) :type maxResults: int :rtype: ResultList """

        params = {
            'username': username,
            'projectKeys': projectKeys}
        return self._fetch_pages(User, None, 'user/assignable/multiProjectSearch', startAt, maxResults, params)