The Free On-line Dictionary of Computing (30 December 2018):

Basic Object System

    (BOS) A C-callable library that implements the
   notion of object and which uses Tcl as its interpreter
   for interpreted methods (you can have "compiled" methods in
   C, and mix compiled and interpreted methods in the same
   object, plus lots more).  You can subclass and mix in existing
   objects using BOS to extend, among other things, the set of
   tk widgets.  BOS is a class-free object system, also
   called a prototype-based object system; it is modelled loosely
   on the Self system from Stanford University.

   Version 1.31 by Sean Levy .

   (ftp://barkley.berkeley.edu/tcl).

   (1992-08-21)