anchor.diff:  	else
   		L = corners - effect_str
   		for (thing in L) // New corners
  -			if(!thing)
  -				continue
   			C = thing
   			LAZYADD(C.affecting, src)
   			if (!C.active)
  
positive.code: 			APPLY_CORNER(C)
  
  		for (thing in corners - L) // Existing corners
  			if(!thing)
  				continue
  			C = thing
  			if (!C.active)
  				effect_str[C] = 0
  
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)
  