Files for constructing universe.ss: 

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

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

 timer.ss              the clock-mixin

 check-aux.ss          common primitives 
 image.ss              the world image functions 
 syn-aux.ss            syntactic auxiliaries 
 syn-aux-aux.ss        auxiliaries to the syntactic auxiliaries