sentences:  Via the task-switcher API, graphical user interfaces such as ViewMAX or PC/GEOS could register as the task manager menu and thereby replace the TASKMAX text mode menu, so that users could switch between tasks from within a GUI.Microsoft responded with MS-DOS 6.0, which again matched some features of DR DOS 6.0.In April 1991, a pre-release version of Windows 3.1 was found to return a non-fatal error message if it detected a non-Microsoft DOS.