<SYSTEM_TASK:>
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_issues(self, username, project=None, issueKey=None, expand=None, startAt=0, maxResults=50, ):
 """Get a list of user Resources that match the search string for assigning or creating issues. This method is intended to find users that are eligible to create issues in a project or be assigned to an existing issue. When searching for eligible creators, specify a project. When searching for eligible assignees, specify an issue key. :param username: A string to match usernames against :type username: str :param project: Filter returned users by permission in this project (expected if a result will be used to create an issue) :type project: Optional[str] :param issueKey: Filter returned users by this issue (expected if a result will be used to edit this issue) :type issueKey: Optional[str] :param expand: Extra information to fetch inside each resource :type expand: Optional[Any] :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 items in batches. (Default: 50) :rtype: ResultList """

        params = {
            'username': username}
        if project is not None:
            params['project'] = project
        if issueKey is not None:
            params['issueKey'] = issueKey
        if expand is not None:
            params['expand'] = expand
        return self._fetch_pages(User, None, 'user/assignable/search', startAt, maxResults, params)