The Vocabulary system is a list of words and their translations which is stored and provides some extended functions.
This vocabulary system is sufficient to record a set of pairs of words, each pair comprising a word in your native language and a corresponding word in the foreign language. This is the kind of information as provided by a very simple pocket bilingual dictionary. Using this system, you can look up a word in the native language section to find which word or words can be used as translations of that native word, and similarly a foreign word can be looked up to discover its possible native language.
This vocabulary system also imposes a constraint upon the words which may be entered into the vocabulary. This constraints imposed by the orthographic rules will be represented by requiring that words of the language belong to two subsets OrthoNative and OrthoForeign, which provide a spelling-check function.
This system will also maintain a record of progress for the student, based on vocabulary tests so that the student may be tested on the current vocabulary; native to foreign or foreign to native. In the native to foreign case, the system choose a native word, and the student choose as many as possible of the known foreign translations, and the the system scores the correct answer; in the foreign to native case, the roles of the languages are reversed.