Index: src/uk/me/parabola/mkgmap/osmstyle/RuleIndex.java
===================================================================
--- src/uk/me/parabola/mkgmap/osmstyle/RuleIndex.java	(revision 4179)
+++ src/uk/me/parabola/mkgmap/osmstyle/RuleIndex.java	(working copy)
@@ -27,6 +27,7 @@
 import uk.me.parabola.mkgmap.osmstyle.eval.Op;
 import uk.me.parabola.mkgmap.reader.osm.Rule;
 import uk.me.parabola.mkgmap.reader.osm.TagDict;
+import uk.me.parabola.util.MultiHashMap;
 
 /**
  * An index to reduce the number of rules that have to be executed.
@@ -171,6 +172,12 @@
 		// Maps a rule number to the tags that might be changed by that rule
 		Map<Integer, List<String>> changeTags = new HashMap<Integer, List<String>>();
 		
+		// Collection of possibly changed or added tags were the new value is NOT known (e.g. set x=$y)
+		Set<String> modOrAddedTagKeys = new HashSet<>();
+		
+		// Collection of possibly changed or added tags were the new value is known (e.g. set x=1)
+		MultiHashMap<String, String> modOrAddedTags = new MultiHashMap<>(); 
+		
 		// remove unnecessary rules
 		filterRules();
 		
@@ -188,10 +195,27 @@
 				int ind = keystring.indexOf('=');
 				if (ind >= 0) {
 					String key = keystring.substring(0, ind);
-					addNumberToMap(tagnames, key, ruleNumber);
+					boolean needed = (modOrAddedTagKeys.contains(key));
+					if (!needed) {
+						List<String> values = modOrAddedTags.get(key);
+						if (values.contains("") || values.contains(keystring.substring(ind+1)))
+								needed = true;
+					}
+					if (needed) {
+						addNumberToMap(tagnames, key, ruleNumber);
+					}
 				} 
 			}
 			addChangables(changeTags, changeableTags, ruleNumber);
+			for (String ent : changeableTags) {
+				int ind = ent.indexOf('=');
+				if (ind >= 0) {
+					modOrAddedTags.add(ent.substring(0, ind), ent.substring(ind+1));
+				} else {
+					modOrAddedTagKeys.add(ent);
+					modOrAddedTags.add(ent,""); // empty string means value not known
+				}
+			}
 		}
 		
 		for (Map.Entry<Integer, List<String>> ent : changeTags.entrySet()) {
@@ -358,6 +382,8 @@
 	 * @param ruleNumber The rule number.
 	 */
 	private static void addChangables(Map<Integer, List<String>> changeTags, Set<String> changeableTags, int ruleNumber) {
+		if(changeableTags.isEmpty())
+			return;
 		List<String> tags = changeTags.get(ruleNumber);
 		if (tags == null) {
 			tags = new ArrayList<String>();