DES version 6.6 (released on September, 4th, 2021) DES version 6.5 (released on April, 10th, 2020) DES version 6.4 (released on January, 20th, 2020) DES version 6.3 (released on October, 14th, 2019) DES version 6.2 (released on January, 21st, 2019) DES version 6.1 (released on May, 24th, 2018) DES version 6.0 (released on April, 9th, 2018) Underlying Alternative Prolog Systems: | 
