diff --git a/Dockerfile b/Dockerfile
index 773912d..534fd40 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -10,6 +10,16 @@ ENV PUB_CACHE=/cache/pub-cache
 ENV GRADLE_USER_HOME=/cache/gradle-cache
 RUN mkdir -p $PUB_CACHE $GRADLE_USER_HOME
 
+# To avoid: fatal: detected dubious ownership in repository at '/sdks/flutter'
+# Pass this during build: --build-arg USER_ID=$(id -u) --build-arg GROUP_ID=$(id -g)
+ARG USER_ID
+ARG GROUP_ID
+RUN echo "Using UID: $USER_ID and GID: $GROUP_ID"
+RUN chown -R $USER_ID:$GROUP_ID $PUB_CACHE $GRADLE_USER_HOME
+RUN chown -R $USER_ID:$GROUP_ID /sdks/flutter
+
+USER $USER_ID:$GROUP_ID
+
 # Copy pubspec files and get dependencies
 # Commented out to avoid building the app during image creation, this will be handled externally by makefile
 # COPY pubspec.yaml pubspec.lock ./
diff --git a/Makefile b/Makefile
index 287ab74..a2e95d6 100644
--- a/Makefile
+++ b/Makefile
@@ -19,11 +19,14 @@ build-linux-debug-container: pub-get-container
 build-web-debug-container: pub-get-container
 	./docker-exec.sh exec flutter build web --debug
 
-run-linux-debug-container: build-linux-debug-container
+run-linux-debug-container: pub-get-container
 	./docker-exec.sh exec flutter run -d linux
 
-run-linux-debug: build-linux-debug-container
+run-web-debug-container: pub-get-container
+	./docker-exec.sh exec flutter run --web-port $${WEB_PORT:-8081} -d web-server
+
+run-linux-debug-native: build-linux-debug-container
 	./build/linux/x64/debug/bundle/d4rt_formulas
 
-run-web-debug-container: build-web-debug-container
-	cd build/web && python3 -m http.server 8080
+run-web-debug-native: build-web-debug-container
+	cd build/web && python3 -m http.server $${WEB_PORT:-8081}
diff --git a/assets/formulas/formulas.d4rt b/assets/formulas/formulas.d4rt
index 67ee938..7291928 100644
--- a/assets/formulas/formulas.d4rt
+++ b/assets/formulas/formulas.d4rt
@@ -132,9 +132,9 @@ Where:
       {"name": "Reflexes", "values": ["No response", "Grimace on aggressive stimulation", "Cry on stimulation"] },
       {"name": "SkinColor", "values": ["Blue or pale", "Blue extremities, pink body", "Pink"] }
     ],
-    "output": {"name": "Result", "unit": "scalar"},
+    "output": {"name": "Result", "unit": "string"},
     "d4rtCode": """
-      var total = HeartRate + Breathing + MuscleTone + Reflexes + SkinColor;
+      var total = indexOf("HeartRate") + indexOf("Breathing") + indexOf("MuscleTone") + indexOf("Reflexes") + indexOf("SkinColor");
       late var interpretation;
       if( total < 4 ) {
         interpretation = 'Critical condition';
@@ -154,12 +154,12 @@ Where:
     "name": "Compare price per mass",
     "description": "Compares two products by their price per mass and returns which is cheaper.",
     "input": [
-      {"name": "price1", "unit": "scalar"},
+      {"name": "price1", "unit": "currency"},
       {"name": "mass1", "unit": "kilogram"},
-      {"name": "price2", "unit": "scalar"},
+      {"name": "price2", "unit": "currency"},
       {"name": "mass2", "unit": "kilogram"}
     ],
-    "output": {"name": "Result", "unit": "scalar"},
+    "output": {"name": "Result", "unit": "string"},
     "d4rtCode": """
       var p1 = price1 / mass1;
       var p2 = price2 / mass2;
diff --git a/assets/units/currency.d4rt.units b/assets/units/currency.d4rt.units
new file mode 100644
index 0000000..82a454f
--- /dev/null
+++ b/assets/units/currency.d4rt.units
@@ -0,0 +1,3 @@
+[
+  {"name": "currency", "symbol": "¤", "isBase": true},
+]
\ No newline at end of file
diff --git a/assets/units/scalar.d4rt.units b/assets/units/scalar.d4rt.units
index 0533d87..a235aff 100644
--- a/assets/units/scalar.d4rt.units
+++ b/assets/units/scalar.d4rt.units
@@ -1,3 +1,4 @@
 [
-  {"name": "scalar", "symbol": "", "isBase": true},
+  {"name": "scalar", "symbol": "㊷", "isBase": true},
+  {"name": "string", "symbol": "🔤", "isBase": true}
 ]
\ No newline at end of file
diff --git a/docker-exec.sh b/docker-exec.sh
index 664b007..b5599ac 100755
--- a/docker-exec.sh
+++ b/docker-exec.sh
@@ -19,9 +19,12 @@ detect_container(){
     fi
 }
 
+clean_build_cache(){
+  $DOCKER builder prune --all --force
+}
 
 build_image(){
-    $DOCKER build -t d4rt-formulas-builder -f Dockerfile .
+    $DOCKER build --build-arg USER_ID=$(id -u) --build-arg GROUP_ID=$(id -g) --progress=plain -t d4rt-formulas-builder -f Dockerfile .
 }
 
 graphic_options(){
@@ -45,19 +48,37 @@ graphic_options(){
     fi
 }
 
-exec_in_container(){
-    local SPIOPTIONS="--env AT_SPI_BUS=/run/user/$(id -u)/at-spi/bus_0 --volume=/run/user/$(id -u)/at-spi:/run/user/$(id -u)/at-spi  --device=/dev/dri"
+spi_options(){
+    if [ -e /run/user/$(id -u)/at-spi/bus_0 ]
+    then
+      printf " %s " "--env AT_SPI_BUS=/run/user/$(id -u)/at-spi/bus_0"
+    fi
 
+    if [ -e /run/user/$(id -u)/at-spi ]
+     then
+      printf " %s " "--volume=/run/user/$(id -u)/at-spi:/run/user/$(id -u)/at-spi"
+    fi
+    if [ -e /dev/dri ]
+    then
+      printf " %s " "--device /dev/dri"
+    fi
+}
+
+exec_in_container(){
+    SPIOPTIONS=$(spi_options)
     local GRAPHICOPTIONS=$(graphic_options)
     local BUILDCACHE=./.build-container-cache
     mkdir -p $BUILDCACHE
 
     $DOCKER run \
             -it \
+            --userns=keep-id \
+            --user $(id -u):$(id -g) \
             --init \
             --rm \
             $GRAPHICOPTIONS \
             $SPIOPTIONS \
+            -p ${WEBPORT:-8081}:8081 \
             -v $BUILDCACHE:/cache:z \
             -v .:/app:z \
             -e FLUTTER_FLAVOR=prod \
@@ -73,12 +94,18 @@ main(){
         return $?
     fi
 
+    if [ "$1" = "cleancache" ]; then
+        clean_build_cache
+        return $?
+    fi
+
+
     if [ "$1" = "exec" ]; then
         exec_in_container ${@:2}
         return $?
     fi
 
-    echo "Usage: $0 {build|exec <command>}"
+    echo "Usage: $0 {build|cleancache|exec <command>}"
     return 1
 }
 
diff --git a/lib/ai/formula_screen.dart b/lib/ai/formula_screen.dart
index 5324954..6a35b9c 100644
--- a/lib/ai/formula_screen.dart
+++ b/lib/ai/formula_screen.dart
@@ -94,7 +94,6 @@ class _FormulaScreenState extends State<FormulaScreen> {
 
   void _evaluateFormula() {
     print( "EVALUATE FORMULA");
-    if (!_formKey.currentState!.validate()) return;
 
     try {
       final inputValues = <String, dynamic>{};
@@ -145,7 +144,7 @@ class _FormulaScreenState extends State<FormulaScreen> {
 
       // Convert output to selected unit if needed
       String? unit = widget.formula.output.unit;
-      if (unit != null && unit is Number) {
+      if (unit != null && result is Number) {
         final converted = widget.corpus.convert(result, unit, _selectedOutputUnit!);
         if (converted is num) {
           _result = converted.toStringAsFixed(2);
@@ -274,6 +273,7 @@ class _FormulaScreenState extends State<FormulaScreen> {
               onUnitChanged: (unit) {
                 _selectedOutputUnit = unit;
                 _evaluateFormula();
+                print( "En output unit changed to $unit: $_result");
                 setState(() {
                 });
               },
diff --git a/lib/defaults/default_corpus.dart b/lib/defaults/default_corpus.dart
index af7f983..52d84f2 100644
--- a/lib/defaults/default_corpus.dart
+++ b/lib/defaults/default_corpus.dart
@@ -20,6 +20,7 @@ Future<Corpus> createDefaultCorpus() async{
     final unitResources = [
       "assets/units/angle.d4rt.units",
       "assets/units/area.d4rt.units",
+      "assets/units/currency.d4rt.units",
       "assets/units/distance.d4rt.units",
       "assets/units/energy.d4rt.units",
       "assets/units/force.d4rt.units",
@@ -32,6 +33,7 @@ Future<Corpus> createDefaultCorpus() async{
     ];
 
     for (final unitRes in unitResources) {
+      print( "Loading units from $unitRes");
       final literal = await loadResourceAsString(unitRes);
       final units = UnitSpec.fromArrayStringLiteral(literal);
       corpus.loadUnits(units);
diff --git a/lib/formula_evaluator.dart b/lib/formula_evaluator.dart
index 0532ee2..40e67f7 100644
--- a/lib/formula_evaluator.dart
+++ b/lib/formula_evaluator.dart
@@ -104,8 +104,9 @@ class FormulaEvaluator {
       final result = _interpreter.execute(source: completeSource);
       return result;
     }
-    catch (e) {
+    catch (e, stack) {
       print( "Error evaluating formula source:\n$completeSource" );
+      print( stack );
       throw FormulaEvaluationException(
         'Error evaluating formula "${formula.name}": $e',
         e,
@@ -141,6 +142,8 @@ class FormulaEvaluator {
       import "package:d4rt_formulas.dart";
   """;
 
+  static const reservedVariableNames = { "variableValues", "indexOf", "variableAllowedValues"} ;
+
   String _buildCompleteSource(Formula formula, Map<String, dynamic> inputValues) {
     final buffer = StringBuffer();
 
@@ -168,6 +171,29 @@ class FormulaEvaluator {
         """);
       }
     }
+
+    buffer.writeln("""
+          final variableValues = <String, dynamic>{
+    """);
+    for (final entry in inputValues.entries) {
+      final varName = entry.key;
+      final value = entry.value;
+
+      if (value is String) {
+        final escapedValue = value.replaceAll('"', '\\"');
+        buffer.writeln("""
+          "$varName": "$escapedValue",
+        """);
+      } else {
+        buffer.writeln("""
+          "$varName": "$value",
+        """);
+      }
+    }
+    buffer.writeln("""
+          };
+    """);
+
     // Build a Map<String, List<String>> named `variableValues` that exposes allowed values
     // for each VariableSpec (inputs and output) to the interpreted code. Values are
     // converted to strings and quoted in the produced d4rt source.
@@ -187,13 +213,24 @@ class FormulaEvaluator {
     }
 
     // Write the variableValues map into the generated source without escaping names/values
-    buffer.writeln("final variableValues = {");
+    buffer.writeln("final variableAllowedValues = {");
     variableValuesMap.forEach((name, list) {
       final listLiteral = list.map((s) => '"' + s + '"').join(', ');
       buffer.writeln('  "' + name + '": [' + listLiteral + '],');
     });
     buffer.writeln('};');
 
+    // Some functions to deal with string values
+    buffer.writeln("""
+      // If return type is int, there is an error converting double to int 🤷‍
+      dynamic indexOf(String inputName) {
+        String value = variableValues[inputName];
+        String allowedValues = variableAllowedValues[inputName];
+        dynamic ret = allowedValues.indexOf(value) as int;
+        return ret as int;
+      }
+      """);
+
     buffer.writeln("""
       late var ${formula.output.name};
       ${formula.d4rtCode}
diff --git a/lib/formula_models.dart b/lib/formula_models.dart
index 9a0c5f6..bff597c 100644
--- a/lib/formula_models.dart
+++ b/lib/formula_models.dart
@@ -1,5 +1,6 @@
 import 'package:d4rt/d4rt.dart';
 import 'package:collection/collection.dart';
+import 'package:d4rt_formulas/d4rt_formulas.dart';
 
 abstract class SetUtils {
   static Object safeGet(Map<Object?, Object?> map, String key) {
@@ -105,6 +106,13 @@ class VariableSpec {
   final List<dynamic>? values;
 
   VariableSpec({required this.name, this.unit, this.values}){
+    validate();
+  }
+
+  void validate(){
+    if( FormulaEvaluator.reservedVariableNames.contains(name) ){
+      throw ArgumentError("$name: is a reserved variable name for FormulaEvaluator");
+    }
     final valuesValid = values != null && values?.isNotEmpty == true;
     if( unit == null && !valuesValid ){
       throw ArgumentError("$name: at least unit or allowedValues should be valid");