Files for constructing universe.rkt:

 world.rkt              the old world
                        world%  = (clock-mixin ...) -- the basic world
                        aworld% = (class world% ...) -- the world with recording

 universe.rkt           the universe server
                        universe% = (clock-mixin ...) -- the basic universe

 timer.rkt              the clock-mixin

 check-aux.rkt          common primitives
 image.rkt              the world image functions
 clauses-spec-and-process.rkt syntactic auxiliaries
 clauses-spec-aux.rkt        auxiliaries to the syntactic auxiliaries
