>=0  ==> SUCCESS
-1   ==> CONNECTION_FAILED 
-2   ==> DOCUMENT_ALREADY_LOADED 
-3   ==> DOCUMENT_DOES_NOT_EXIST
-4   ==> EDITING_OUTSIDE_BOUNDS
-5   ==> CANNOT_EDIT_MULTIPLE_LINES
-6   ==> CANNOT_LOCK
-7   ==> CANNOT_UNLOCK
-8   ==> LINE_DOES_NOT_EXIST