a system for retrieving information units from a data structure . the system includes a search engine and an on - screen user interface . the data structure includes a dictionary file having keywords related to each other by a hierarchy , wherein each information unit is associated with a keyword . the user interface provides a text input zone to define a search and a user control to order the search engine to perform the search depending on the contents of the text input zone . the user interface also provides a first on - screen list display zone for showing results of the search performed by the search engine and a second list display zone for showing the results from the hierarchy related to the results shown in the first display zone .