Update multiz_helpers.dm

code/modules/mapping/space_management/multiz_helpers.dm

@@ -34,14 +34,10 @@
 	return get_step_multiz(src, DOWN)
 
 /proc/dir_inverse_multiz(dir)
-	var/holder = dir & (UP|DOWN)
-	if((holder == NONE) || (holder == (UP|DOWN)))
-		return turn(dir, 180)
-	dir &= ~(UP|DOWN)
-	dir = turn(dir, 180)
-	if(holder == UP)
-		holder = DOWN
-	else
-		holder = UP
-	dir |= holder
-	return dir
+	var/vertical = dir & (UP|DOWN)
+	dir &= ~vertical	// remove dir from vertical
+	if(vertical == (UP|DOWN))		// both up/down specified
+		vertical = NONE
+	if(dir)		// byond will be a shit if dir is 0 so let's not have dir be 0.
+		dir = turn(dir, 180)
+	return dir | vertical