anchor.diff:  			APPLY_CORNER(C)
   
   		for (thing in corners - L) // Existing corners
  -			if(!thing)
  -				continue
   			C = thing
   			if (!C.active)
   				effect_str[C] = 0
  
positive.code: 
  	L = effect_str - corners
  	for (thing in L) // Old, now gone, corners.
  		if(!thing)
  			continue
  		C = thing
  		REMOVE_CORNER(C)
  		LAZYREMOVE(C.affecting, src)
  
negative.code: 	LAZYINITLIST(effect_str)
  	if (needs_update == LIGHTING_VIS_UPDATE)
  		for (thing in  corners - effect_str) // New corners
  			C = thing
  			LAZYADD(C.affecting, src)
  			if (!C.active)
  