return the symbols used in the knowledge base clauses concode_field_sep ArrayList<Sentence> sentences concode_field_sep List<Clause> getClauses concode_elem_sep void tell concode_elem_sep void tell concode_elem_sep List<Sentence> getSentences
List < PropositionalSymbol > function ( ) { return sentences . stream ( ) . flatMap ( Sentence :: getPropositionalSymbols ) . collect ( Collectors . toList ( ) ) ; }