  SASyLF was released in 2008 and used as a proof assistant in courses at
several universities. It proved itself useful and has continued to be used, and
each iteration of use has encouraged further development: fixing bugs and
adding enhancements. This paper describes how SASyLF was developed while
keeping true to its purpose. Most notable are making substitutions explicit,
support of "and" and "or," support for mutual and lexicographic induction, and
IDE support.
