Source : Free On-Line Dictionary of Computing

Portable Scheme Interpreter
     
        (PSI) A portable scheme interpreter by Ozan Yigit
        , David Keldsen and Pontus Hedman that
        includes a simple {DAG} compiler and a {virtual machine}.  It
        can be used as an integrated extension {interpreter} in other
        systems and allows easy addition of new primitives.  There are
        some unique debugging and tracing facilities.  Acceptable
        performance results from a fairly straight-forward
        implementation.  {Continuation}s are fully and portably
        supported and perform well.  PSI is based on the simple
        compilers and {virtual machine} in Kent Dbyvig's thesis.
     
        The pre-release version conforms to {R4RS} with a number of
        useful extensions.
     
        (1993-02-19)