v4.0.1

Minor update

v4.0.0

• Fix bugs
• Improve reliability

v3.1.0

Robustness Improvement

v3.0.2

Fix minor bugs

v3.0.1

fix bugs

v3.0.0

• reduce memory usage
• distinguish memory-optimization mode between disk-optimization mode

v2.4.0

• check backup before quiz
• backup in the same dir
• remove -1 option
• update manpage
• fix bugs

v2.3.1

fix typos and imprecise description in the man page

v2.3.0

• quiz flashcards in the same order in input files
• fix a bug in exemod()
• improve performance
• update the man page

v2.2.1

• Fix a bug in exemod();
• Fix bad indent in backup();
• Update README.md.