<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def search_allowed_users_for_issue(self, user, issueKey=None, projectKey=None, startAt=0, maxResults=50):
 """Get a list of user Resources that match a username string and have browse permission for the issue or project. :param user: a string to match usernames against. :type user: str :param issueKey: find users with browse permission for this issue. :type issueKey: Optional[str] :param projectKey: find users with browse permission for this project. :type projectKey: Optional[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 items in batches. (Default: 50) :type maxResults: int """

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