Grammarians have sometimes tried to extend the subject predicatedefinition by assuming ellipsis in sentences in which the subject or predicate or both are lacking. But we can safelyassume ellipsis only when there is no doubt about what has beenomitted. It is not conspicuously unreasonable to say that in animperative sentence like ''Shut the door" there is an ellipsis ofthe subject you. But what is the ellipsis in* 'Heads up" ? There areseveral possibilities: