Eckstut-Didier, S. (1994). Focus on grammar: Workbook. A basic course for reference and practice (1a. ed.). Addison-Wesley.

Eckstut-Didier, Samuela. Focus on Grammar: Workbook. A Basic Course for Reference and Practice. 1a. ed. Reading: Addison-Wesley, 1994.

Eckstut-Didier, Samuela. Focus on Grammar: Workbook. A Basic Course for Reference and Practice. 1a. ed. Addison-Wesley, 1994.