diff --git a/README.md b/README.md
index e7685f0..67c244b 100644
--- a/README.md
+++ b/README.md
@@ -29,9 +29,8 @@ The file is a json array of formulas.
                 "magnitude" : "Force"
             }
         },
-        "d4rt_code":{
-            "return m*a;"
-        }
+        "d4rt_code": "return m*a;"
+        
     }
 
 ]
diff --git a/bin/pruebas_d4rt.dart b/bin/pruebas_d4rt.dart
index 15cc375..f9da874 100644
--- a/bin/pruebas_d4rt.dart
+++ b/bin/pruebas_d4rt.dart
@@ -1,4 +1,4 @@
-import 'package:pruebas_d4rt/pruebas_d4rt.dart' as pruebas_d4rt;
+import 'package:d4rt_formulas/d4rt_formulas.dart' as pruebas_d4rt;
 import 'package:d4rt/d4rt.dart';
 
 void main() {
diff --git a/example/formula_evaluation_example.dart b/example/formula_evaluation_example.dart
index 4f0ce55..333a3ce 100644
--- a/example/formula_evaluation_example.dart
+++ b/example/formula_evaluation_example.dart
@@ -17,13 +17,11 @@ void main() {
   print('1. Newton\'s Second Law of Motion');
   final newtonFormula = Formula(
     name: "Newton's Second Law",
-    input: {
-      'm': VariableSpec(magnitude: 'mass'),
-      'a': VariableSpec(magnitude: 'acceleration'),
-    },
-    output: {
-      'F': VariableSpec(magnitude: 'force'),
-    },
+    input: [
+      VariableSpec(name: 'm', magnitude: 'mass'),
+      VariableSpec(name: 'a', magnitude: 'acceleration'),
+    ],
+    output:  VariableSpec(name: 'F', magnitude: 'force'),
     d4rtCode: '''
       main() {
         return m * a;
@@ -52,14 +50,12 @@ void main() {
   print('2. Quadratic Formula Discriminant (Δ = b² - 4ac)');
   final discriminantFormula = Formula(
     name: 'Quadratic Discriminant',
-    input: {
-      'a': VariableSpec(magnitude: 'coefficient'),
-      'b': VariableSpec(magnitude: 'coefficient'),
-      'c': VariableSpec(magnitude: 'coefficient'),
-    },
-    output: {
-      'discriminant': VariableSpec(magnitude: 'scalar'),
-    },
+    input: [
+      VariableSpec(name: 'a', magnitude: 'coefficient'),
+      VariableSpec(name: 'b', magnitude: 'coefficient'),
+      VariableSpec(name: 'c', magnitude: 'coefficient'),
+    ],
+    output : VariableSpec(name: 'discriminant', magnitude: 'scalar'),
     d4rtCode: '''
       main() {
         return b * b - 4 * a * c;
@@ -95,12 +91,11 @@ void main() {
   print('3. Circle Area (A = π * r²)');
   final circleAreaFormula = Formula(
     name: 'Circle Area',
-    input: {
-      'r': VariableSpec(magnitude: 'length'),
-    },
-    output: {
-      'A': VariableSpec(magnitude: 'area'),
-    },
+    input: [
+      VariableSpec(name: 'r', magnitude: 'length'),
+    ],
+    output: VariableSpec(name: 'A', magnitude: 'area'),
+
     d4rtCode: '''
       main() {
         var pi = 3.14159265359;
@@ -141,15 +136,13 @@ void main() {
   print('5. Compound Interest Formula');
   final compoundInterestFormula = Formula(
     name: 'Compound Interest',
-    input: {
-      'P': VariableSpec(magnitude: 'currency'),      // Principal
-      'r': VariableSpec(magnitude: 'rate'),          // Annual interest rate
-      'n': VariableSpec(magnitude: 'count'),         // Times compounded per year
-      't': VariableSpec(magnitude: 'time'),          // Time in years
-    },
-    output: {
-      'A': VariableSpec(magnitude: 'currency'),      // Final amount
-    },
+    input: [
+      VariableSpec(name: 'P', magnitude: 'currency'),      // Principal
+      VariableSpec(name: 'r', magnitude: 'rate'),          // Annual interest rate
+      VariableSpec(name: 'n', magnitude: 'count'),         // Times compounded per year
+      VariableSpec(name: 't', magnitude: 'time'),          // Time in years
+    ],
+    output: VariableSpec(name: 'A', magnitude: 'currency'),      // Final amount
     d4rtCode: '''
       main() {
         // A = P * (1 + r/n)^(n*t)
diff --git a/lib/formula_evaluator.dart b/lib/formula_evaluator.dart
index d1bd957..65e3a89 100644
--- a/lib/formula_evaluator.dart
+++ b/lib/formula_evaluator.dart
@@ -1,11 +1,3 @@
-/// Formula evaluator that executes d4rt code with input variables and
-/// returns the value of the single output variable.
-///
-/// The evaluator assumes that:
-/// - A formula has exactly one output variable
-/// - The d4rt code defines a main function with parameters matching input variables
-/// - The d4rt code when executed returns the value of that output variable
-/// - Input variables are provided as a Map<String, dynamic>
 
 import 'package:d4rt/d4rt.dart';
 import 'formula_models.dart';
@@ -73,7 +65,7 @@ class FormulaEvaluator {
   void _validateInputValues(Formula formula, Map<String, dynamic> inputValues) {
     final missingVars = <String>[];
     
-    for (final inputVar in formula.input.keys) {
+    for (final inputVar in formula.inputVarNames()) {
       if (!inputValues.containsKey(inputVar)) {
         missingVars.add(inputVar);
       }
@@ -89,19 +81,18 @@ class FormulaEvaluator {
 
   /// Gets the name of the single output variable from the formula
   String getOutputVariableName(Formula formula) {
-    // Formula construction already ensures exactly one output variable
-    return formula.output.keys.first;
+    return formula.output.name;
   }
 
   /// Gets the magnitude of the single output variable from the formula
   String getOutputVariableMagnitude(Formula formula) {
     // Formula construction already ensures exactly one output variable
-    return formula.output.values.first.magnitude;
+    return formula.output.magnitude;
   }
 
   /// Gets the ordered list of input variable names (alphabetically sorted)
   List<String> getInputVariableOrder(Formula formula) {
-    return formula.input.keys.toList()..sort();
+    return formula.inputVarNames()..sort();
   }
 
   /// Builds the complete d4rt source code by injecting variable declarations
diff --git a/lib/formula_models.dart b/lib/formula_models.dart
index a7f0f5e..8d48cc8 100644
--- a/lib/formula_models.dart
+++ b/lib/formula_models.dart
@@ -1,49 +1,33 @@
-/// Data classes to represent a formula and its I/O specs.
-///
-/// Structure (from README.md):
-/// - name: String
-/// - input: { varName: { magnitude: String } }
-/// - output: { varName: { magnitude: String } }
-/// - d4rt_code: { code: String } or String
-///
-/// - Accept d4rt_code as either a plain string or an object { code: "..." }.
+
+import 'package:d4rt/d4rt.dart';
 
 class VariableSpec {
+
+  final String name;
   final String magnitude;
+  static final MAGNITUDELESS = "magnitudeless";
 
-  const VariableSpec({required this.magnitude});
-
-  factory VariableSpec.fromJson(Map<String, dynamic> json) {
-    final mag = json['magnitude'];
-    if (mag is! String || mag.trim().isEmpty) {
-      throw FormatException("'magnitude' must be a non-empty string");
-    }
-    return VariableSpec(magnitude: mag);
-  }
-
-  Map<String, dynamic> toJson() => {'magnitude': magnitude};
+  VariableSpec({required this.name, required this.magnitude});
 
   @override
-  String toString() => 'VariableSpec(magnitude: $magnitude)';
-
-  VariableSpec copyWith({String? magnitude}) =>
-      VariableSpec(magnitude: magnitude ?? this.magnitude);
+  String toString() => 'var($name: $magnitude)';
 
   @override
   bool operator ==(Object other) =>
       identical(this, other) ||
       other is VariableSpec &&
           runtimeType == other.runtimeType &&
-          magnitude == other.magnitude;
+          magnitude == other.magnitude &&
+          name == other.name;
 
   @override
-  int get hashCode => magnitude.hashCode;
+  int get hashCode => Object.hash(magnitude, name);
 }
 
 class Formula {
   final String name;
-  final Map<String, VariableSpec> input; // Supports multiple input variables
-  final Map<String, VariableSpec> output; // Supports multiple output variables
+  final List<VariableSpec> input;
+  final VariableSpec output;
   final String d4rtCode;
 
   Formula({
@@ -59,116 +43,27 @@ class Formula {
     if (name.trim().isEmpty) {
       throw ArgumentError('Formula name cannot be empty');
     }
-    if (output.length != 1) {
-      throw ArgumentError(
-        'Formula "$name" must have exactly one output variable, '
-        'but has ${output.length}',
-      );
-    }
   }
 
-  factory Formula.fromJson(Map<String, dynamic> json) {
-    final name = json['name'];
-    if (name is! String || name.trim().isEmpty) {
-      throw FormatException('Formula requires a non-empty name string');
-    }
-
-    Map<String, VariableSpec> parseVars(dynamic obj, String fieldName) {
-      if (obj == null) return <String, VariableSpec>{};
-      if (obj is! Map<String, dynamic>) {
-        throw FormatException('$fieldName must be an object map');
-      }
-      return obj.map((k, v) {
-        if (v is! Map<String, dynamic>) {
-          throw FormatException(
-            'Variable "$k" in $fieldName must be an object',
-          );
-        }
-        return MapEntry(k, VariableSpec.fromJson(v));
-      });
-    }
-
-    String parseCode(dynamic code) {
-      if (code == null) {
-        throw FormatException('d4rt_code is required');
-      }
-      if (code is String) return code;
-      if (code is Map<String, dynamic>) {
-        final explicit = code['code'];
-        if (explicit is String) return explicit;
-        for (final entry in code.entries) {
-          if (entry.value is String) return entry.value as String;
-        }
-      }
-      throw FormatException(
-        'd4rt_code must be a string or an object containing a code string',
-      );
-    }
-
-    final input = parseVars(json['input'], 'input');
-    final output = parseVars(json['output'], 'output');
-    final d4rtCode = parseCode(json['d4rt_code']);
-
-    return Formula(
-      name: name,
-      input: input,
-      output: output,
-      d4rtCode: d4rtCode,
-    );
-  }
-
-  Map<String, dynamic> toJson() => {
-    'name': name,
-    'input': input.map((k, v) => MapEntry(k, v.toJson())),
-    'output': output.map((k, v) => MapEntry(k, v.toJson())),
-    'd4rt_code': {'code': d4rtCode},
-  };
 
   @override
   String toString() =>
       'Formula(name: $name, input: $input, output: $output, d4rtCode: $d4rtCode)';
 
-  Formula copyWith({
-    String? name,
-    Map<String, VariableSpec>? input,
-    Map<String, VariableSpec>? output,
-    String? d4rtCode,
-  }) => Formula(
-    name: name ?? this.name,
-    input: input ?? this.input,
-    output: output ?? this.output,
-    d4rtCode: d4rtCode ?? this.d4rtCode,
-  );
-
   @override
   bool operator ==(Object other) =>
       identical(this, other) ||
       other is Formula &&
           runtimeType == other.runtimeType &&
           name == other.name &&
-          _mapEquals(input, other.input) &&
-          _mapEquals(output, other.output) &&
+          output == other.output &&
+          ListEquality().equals(input, other.input) &&
           d4rtCode == other.d4rtCode;
 
   @override
   int get hashCode =>
-      Object.hash(name, _mapHash(input), _mapHash(output), d4rtCode);
+      Object.hash(name, ListEquality().hash(input), output, d4rtCode);
+
+  List<String> inputVarNames() => input.map( (v) => v.name ).toList(growable: false);
 }
 
-bool _mapEquals<K, V>(Map<K, V> a, Map<K, V> b) {
-  if (identical(a, b)) return true;
-  if (a.length != b.length) return false;
-  for (final entry in a.entries) {
-    if (!b.containsKey(entry.key)) return false;
-    if (b[entry.key] != entry.value) return false;
-  }
-  return true;
-}
-
-int _mapHash<K, V>(Map<K, V> m) {
-  var h = 0;
-  for (final e in m.entries) {
-    h = h ^ Object.hash(e.key, e.value);
-  }
-  return h;
-}
diff --git a/test/formula_evaluator_test.dart b/test/formula_evaluator_test.dart
index b06bafa..8988853 100644
--- a/test/formula_evaluator_test.dart
+++ b/test/formula_evaluator_test.dart
@@ -14,11 +14,11 @@ void main() {
       test('evaluates Newton\'s second law formula', () {
         final formula = Formula(
           name: "Newton's second law",
-          input: {
-            'm': VariableSpec(magnitude: 'mass'),
-            'a': VariableSpec(magnitude: 'acceleration'),
-          },
-          output: {'F': VariableSpec(magnitude: 'force')},
+          input: [
+            VariableSpec(name: 'm', magnitude: 'mass'),
+            VariableSpec(name: 'a', magnitude: 'acceleration'),
+          ],
+          output: VariableSpec(name: 'F', magnitude: 'force'),
           d4rtCode: '''
             main() {
               return a * m;
@@ -37,11 +37,11 @@ void main() {
       test('evaluates simple arithmetic formula', () {
         final formula = Formula(
           name: 'Simple addition',
-          input: {
-            'x': VariableSpec(magnitude: 'scalar'),
-            'y': VariableSpec(magnitude: 'scalar'),
-          },
-          output: {'result': VariableSpec(magnitude: 'scalar')},
+          input: [
+            VariableSpec(name: 'x', magnitude: 'scalar'),
+            VariableSpec(name: 'y', magnitude: 'scalar'),
+          ],
+          output: VariableSpec(name: 'result', magnitude: 'scalar'),
           d4rtCode: '''
             main() {
               return x + y;
@@ -57,8 +57,8 @@ void main() {
       test('handles single input variable', () {
         final formula = Formula(
           name: 'Square function',
-          input: {'n': VariableSpec(magnitude: 'scalar')},
-          output: {'result': VariableSpec(magnitude: 'scalar')},
+          input: [VariableSpec(name: 'n', magnitude: 'scalar')],
+          output: VariableSpec(name: 'result', magnitude: 'scalar'),
           d4rtCode: '''
             main() {
               return n * n;
@@ -73,12 +73,12 @@ void main() {
       test('handles complex mathematical operations', () {
         final formula = Formula(
           name: 'Quadratic formula discriminant',
-          input: {
-            'a': VariableSpec(magnitude: 'scalar'),
-            'b': VariableSpec(magnitude: 'scalar'),
-            'c': VariableSpec(magnitude: 'scalar'),
-          },
-          output: {'discriminant': VariableSpec(magnitude: 'scalar')},
+          input: [
+            VariableSpec(name: 'a', magnitude: 'scalar'),
+            VariableSpec(name: 'b', magnitude: 'scalar'),
+            VariableSpec(name: 'c', magnitude: 'scalar'),
+          ],
+          output: VariableSpec(name: 'discriminant', magnitude: 'scalar'),
           d4rtCode: '''
             main() {
               return b * b - 4 * a * c;
@@ -96,12 +96,12 @@ void main() {
       test('maintains consistent alphabetical order for input variables', () {
         final formula = Formula(
           name: 'Test order',
-          input: {
-            'z': VariableSpec(magnitude: 'scalar'),
-            'a': VariableSpec(magnitude: 'scalar'),
-            'b': VariableSpec(magnitude: 'scalar'),
-          },
-          output: {'result': VariableSpec(magnitude: 'scalar')},
+          input: [
+            VariableSpec(name: 'z', magnitude: 'scalar'),
+            VariableSpec(name: 'a', magnitude: 'scalar'),
+            VariableSpec(name: 'b', magnitude: 'scalar'),
+          ],
+          output: VariableSpec(name: 'result', magnitude: 'scalar'),
           d4rtCode: 'main() { return a + b + z; }',
         );
 
@@ -112,12 +112,12 @@ void main() {
       test('passes arguments in correct alphabetical order', () {
         final formula = Formula(
           name: 'Test argument order',
-          input: {
-            'z': VariableSpec(magnitude: 'scalar'),
-            'a': VariableSpec(magnitude: 'scalar'),
-            'y': VariableSpec(magnitude: 'scalar'),
-          },
-          output: {'result': VariableSpec(magnitude: 'scalar')},
+          input: [
+            VariableSpec(name: 'z', magnitude: 'scalar'),
+            VariableSpec(name: 'a', magnitude: 'scalar'),
+            VariableSpec(name: 'y', magnitude: 'scalar'),
+          ],
+          output: VariableSpec(name: 'result', magnitude: 'scalar'),
           d4rtCode: '''
             main() {
               // Variables: a=1, y=2, z=3
@@ -133,46 +133,15 @@ void main() {
     });
 
     group('Error handling', () {
-      test(
-        'throws exception for formula with no output variables during construction',
-        () {
-          expect(() {
-            return Formula(
-              name: 'Invalid formula',
-              input: {'x': VariableSpec(magnitude: 'scalar')},
-              output: {}, // No output variables
-              d4rtCode: 'main() { return x; }',
-            );
-          }, throwsA(isA<ArgumentError>()));
-        },
-      );
-
-      test(
-        'throws exception for formula with multiple output variables during construction',
-        () {
-          expect(
-            () => Formula(
-              name: 'Invalid formula',
-              input: {'x': VariableSpec(magnitude: 'scalar')},
-              output: {
-                'y': VariableSpec(magnitude: 'scalar'),
-                'z': VariableSpec(magnitude: 'scalar'),
-              },
-              d4rtCode: 'main() { return x; }',
-            ),
-            throwsA(isA<ArgumentError>()),
-          );
-        },
-      );
 
       test('throws exception for missing input variables', () {
         final formula = Formula(
           name: 'Test formula',
-          input: {
-            'x': VariableSpec(magnitude: 'scalar'),
-            'y': VariableSpec(magnitude: 'scalar'),
-          },
-          output: {'result': VariableSpec(magnitude: 'scalar')},
+          input: [
+            VariableSpec(name: 'x', magnitude: 'scalar'),
+            VariableSpec(name: 'y', magnitude: 'scalar'),
+          ],
+          output: VariableSpec(name: 'result', magnitude: 'scalar'),
           d4rtCode: 'main() { return x + y; }',
         );
 
@@ -185,8 +154,8 @@ void main() {
       test('throws exception for invalid d4rt code', () {
         final formula = Formula(
           name: 'Invalid code formula',
-          input: {'x': VariableSpec(magnitude: 'scalar')},
-          output: {'result': VariableSpec(magnitude: 'scalar')},
+          input: [VariableSpec(name: 'x', magnitude: 'scalar')],
+          output: VariableSpec(name: 'result', magnitude: 'scalar'),
           d4rtCode: 'invalid dart code here!',
         );
 
@@ -201,8 +170,8 @@ void main() {
       test('getOutputVariableName returns the single output variable name', () {
         final formula = Formula(
           name: 'Test',
-          input: {'x': VariableSpec(magnitude: 'scalar')},
-          output: {'force': VariableSpec(magnitude: 'Newton')},
+          input: [VariableSpec(name: 'x', magnitude: 'scalar')],
+          output: VariableSpec(name: 'force', magnitude: 'Newton'),
           d4rtCode: 'main() { return x; }',
         );
 
@@ -214,8 +183,8 @@ void main() {
         () {
           final formula = Formula(
             name: 'Test',
-            input: {'x': VariableSpec(magnitude: 'scalar')},
-            output: {'force': VariableSpec(magnitude: 'Newton')},
+            input: [VariableSpec(name: 'x', magnitude: 'scalar')],
+            output: VariableSpec(name: 'force', magnitude: 'Newton'),
             d4rtCode: 'main() { return x; }',
           );
 
@@ -226,8 +195,8 @@ void main() {
       test('utility methods work correctly with valid formulas', () {
         final validFormula = Formula(
           name: 'Valid Formula',
-          input: {'x': VariableSpec(magnitude: 'scalar')},
-          output: {'result': VariableSpec(magnitude: 'Newton')},
+          input: [VariableSpec(name: 'x', magnitude: 'scalar')],
+          output: VariableSpec(name: 'result', magnitude: 'Newton'),
           d4rtCode: 'main() { return x; }',
         );
 
@@ -235,39 +204,14 @@ void main() {
         expect(evaluator.getOutputVariableMagnitude(validFormula), 'Newton');
       });
 
-      test('validates formula construction with factory method', () {
-        // Test the factory method validation
-        expect(
-          () => Formula(
-            name: 'Invalid',
-            input: {'x': VariableSpec(magnitude: 'scalar')},
-            output: {}, // No output variables
-            d4rtCode: 'main() { return x; }',
-          ),
-          throwsA(isA<ArgumentError>()),
-        );
-
-        expect(
-          () => Formula(
-            name: 'Invalid',
-            input: {'x': VariableSpec(magnitude: 'scalar')},
-            output: {
-              'y': VariableSpec(magnitude: 'scalar'),
-              'z': VariableSpec(magnitude: 'scalar'),
-            },
-            d4rtCode: 'main() { return x; }',
-          ),
-          throwsA(isA<ArgumentError>()),
-        );
-      });
     });
 
     group('Data types', () {
       test('handles integer values', () {
         final formula = Formula(
           name: 'Integer test',
-          input: {'n': VariableSpec(magnitude: 'count')},
-          output: {'result': VariableSpec(magnitude: 'count')},
+          input: [VariableSpec(name: 'n', magnitude: 'count')],
+          output: VariableSpec(name: 'result', magnitude: 'count'),
           d4rtCode: 'main() { return n + 1; }',
         );
 
@@ -278,8 +222,8 @@ void main() {
       test('handles double values', () {
         final formula = Formula(
           name: 'Double test',
-          input: {'x': VariableSpec(magnitude: 'length')},
-          output: {'result': VariableSpec(magnitude: 'area')},
+          input: [VariableSpec(name: 'x', magnitude: 'length')],
+          output: VariableSpec(name: 'result', magnitude: 'area'),
           d4rtCode: 'main() { return x * x; }',
         );
 
diff --git a/test/formula_models_test.dart b/test/formula_models_test.dart
index a77fdcd..95ce74a 100644
--- a/test/formula_models_test.dart
+++ b/test/formula_models_test.dart
@@ -4,71 +4,4 @@ import 'package:test/test.dart';
 import 'package:d4rt_formulas/formula_models.dart';
 
 void main() {
-  group('Formula models', () {
-    test('parses a formula with multiple input variables', () {
-      const jsonStr = '''
-      {
-        "name": "Newton's second law (scalar)",
-        "input": {
-          "m": {"magnitude": "mass"},
-          "a": {"magnitude": "acceleration"}
-        },
-        "output": {
-          "F": {"magnitude": "Force"}
-        },
-        "d4rt_code": {"code": "return m*a;"}
-      }
-      ''';
-
-      final map = jsonDecode(jsonStr) as Map<String, dynamic>;
-      final f = Formula.fromJson(map);
-
-      expect(f.name, "Newton's second law (scalar)");
-      expect(f.input.length, 2);
-      expect(f.input['m']!.magnitude, 'mass');
-      expect(f.input['a']!.magnitude, 'acceleration');
-      expect(f.output.length, 1);
-      expect(f.output['F']!.magnitude, 'Force');
-      expect(f.d4rtCode, 'return m*a;');
-
-      final back = f.toJson();
-      expect(back['name'], f.name);
-      expect((back['input'] as Map)['m']['magnitude'], 'mass');
-      expect((back['input'] as Map)['a']['magnitude'], 'acceleration');
-      expect((back['output'] as Map)['F']['magnitude'], 'Force');
-      expect((back['d4rt_code'] as Map)['code'], 'return m*a;');
-    });
-
-    test('rejects typo magitude (must be magnitude)', () {
-      const badJson = '''
-      {
-        "name": "Bad formula",
-        "input": {
-          "x": {"magitude": "oops"}
-        },
-        "output": {
-          "y": {"magnitude": "ok"}
-        },
-        "d4rt_code": "return x;"
-      }
-      ''';
-
-      final map = jsonDecode(badJson) as Map<String, dynamic>;
-      expect(() => Formula.fromJson(map), throwsFormatException);
-    });
-
-    test('accepts d4rt_code as a plain string', () {
-      const jsonStr = '''
-      {
-        "name": "Simple",
-        "input": {"x": {"magnitude": "scalar"}},
-        "output": {"y": {"magnitude": "scalar"}},
-        "d4rt_code": "return x;"
-      }
-      ''';
-      final map = jsonDecode(jsonStr) as Map<String, dynamic>;
-      final f = Formula.fromJson(map);
-      expect(f.d4rtCode, 'return x;');
-    });
-  });
 }