Abstract. Requirements engineering, the first phase of any software development project, is the Achilles’ heel of the whole development process, as requirements documents are often inconsistent and incomplete. In industrial requirements documents natural language is the main presentation means. In such documents the system behavior is specified in the form of use cases and their scenarios, written as a sequence of sentences in natural language. For the authors of requirements documents some facts are so obvious that they forget to mention them. This surely causes problems for the requirements analyst. Missing information manifests itself, for example, in sentences in passive voice: such sentences just say that some action is performed, but they do not say who performs the action. In the case of requirement analysis this poses a serious problem, as in every real system there is an actor for every performed action. There already exists an approach able to guess missing actors and act...