LFやTwelfの流れを汲む証明系。証明木が見易いのが売りのようです。
[Types ML より] If you're planning to teach a programming language or logic foundations course and are thinking about using a proof assistant, you might consider SASyLF. SASyLF (Second-order Abstract Syntax Logical Framework) is an LF-based proof assistant that the same logical foundation and many of the same advantages as Twelf (in particular, variable binding is "built in"). However, it uses a syntax very close to that used for proofs on paper, giving the tool a much gentler learning curve--and much better error messages--than many alternatives.