diff --git a/README.md b/README.md
index a351e65..8823e1e 100644
--- a/README.md
+++ b/README.md
@@ -8,6 +8,13 @@ It shows how to integrate ANTLR with monaco.
 
 ![Calc Example](./doc/images/calc_example.png)
 
+## Features
+
+* Text editing provided by [monaco](https://github.com/microsoft/monaco-editor)
+* Syntax highlighting for custom language
+* Syntax error reporting with expected alternatives
+* Semantic error reporting for undeclared and redeclared variables
+
 ## Generating the lexer and the parser
 
 ```
diff --git a/src/main/typescript/ParserFacade.ts b/src/main/typescript/ParserFacade.ts
index cc9000a..206eee1 100644
--- a/src/main/typescript/ParserFacade.ts
+++ b/src/main/typescript/ParserFacade.ts
@@ -137,5 +137,65 @@ export function validate(input) : Error[] {
     parser._errHandler = new CalcErrorStrategy();
 
     const tree = parser.compilationUnit();
+
+    const scope = []; // ? Should be a Set but can't use it unless typescript targets ES6+
+
+    for (const input of tree.inputs) {
+
+        if (input.ID() === null) continue;
+
+        const inputIdentifier = input.ID().symbol;
+
+        if (scope.some(x => x === inputIdentifier.text)) {
+            errors.push(createErrorAt(inputIdentifier, "input already declared"));
+        }
+        else {
+            scope.push(inputIdentifier.text);
+        }
+    }
+
+    for (const calculation of tree.calcs) {
+        validateExpression(calculation.value);
+
+        if (!scope.some(x => x === calculation.target.text)) {
+            scope.push(calculation.target.text);
+        }
+    }
+
+    for (const output of tree.outputs) {
+
+        if (output.ID() === null) continue;
+
+        const outputIdentifier = output.ID().symbol;
+
+        if (outputIdentifier.text === "<missing null>") continue;
+
+        if (!scope.some(x => x === outputIdentifier.text)) {
+            errors.push(createErrorAt(outputIdentifier, "undeclared symbol"));
+        }
+    }
+
+    function validateExpression(expression) {
+        if (expression === null || expression.children === null) return;
+
+        for (const child of expression.children) {
+            if (child.children) {
+                validateExpression(child);
+            }
+            else if (child.symbol && child.symbol.type == CalcLexer.ID) {
+
+                const symbol = child.symbol;
+
+                if (!scope.some(x => x == symbol.text)) {
+                    errors.push(createErrorAt(symbol, "undeclared symbol"));
+                }
+            }
+        }
+    }
+
     return errors;
 }
+
+function createErrorAt(node, message) {
+    return new Error(node.line, node.line, node.column + 1, node.column + node.stop - node.start + 2, message);
+}
diff --git a/src/test/javascript/parsingTest.js b/src/test/javascript/parsingTest.js
index 9294653..78731eb 100644
--- a/src/test/javascript/parsingTest.js
+++ b/src/test/javascript/parsingTest.js
@@ -64,12 +64,12 @@ describe('Basic parsing of simple script', function () {
 
 describe('Validation of simple errors on single lines', function () {
     describe('should have recognize missing operand', function () {
-        parseAndCheckErrors("o = i + \n", [
+        parseAndCheckErrors("o = 1 + \n", [
             new parserFacade.Error(1, 1, 8, 9, "mismatched input '\\n' expecting {NUMBER_LIT, ID, '(', '-'}")
         ]);
     });
     describe('should have recognize extra operator', function () {
-        parseAndCheckErrors("o = i +* 2 \n", [
+        parseAndCheckErrors("o = 1 +* 2 \n", [
             new parserFacade.Error(1, 1, 7, 8, "extraneous input '*' expecting {NUMBER_LIT, ID, '(', '-'}")
         ]);
     });
@@ -131,3 +131,65 @@ describe('Unrecognized tokens cause errors', function () {
         ]);
     });
 });
+
+describe('Semantic validation', function () {
+    describe('should report input already declared', function () {
+        let input = "input i\ninput i\n";
+        parseAndCheckErrors(input, [
+            new parserFacade.Error(2, 2, 7, 8, "input already declared")
+        ]);
+    });
+    describe('should report input already declared twice', function () {
+        let input = "input i\ninput i\ninput i\n";
+        parseAndCheckErrors(input, [
+            new parserFacade.Error(2, 2, 7, 8, "input already declared"),
+            new parserFacade.Error(3, 3, 7, 8, "input already declared")
+        ]);
+    });
+    describe('should report undeclared symbol', function () {
+        let input = "o = i\n";
+        parseAndCheckErrors(input, [
+            new parserFacade.Error(1, 1, 5, 6, "undeclared symbol")
+        ]);
+    });
+    describe('should report undeclared symbol in line 4', function () {
+        let input = "input i\no = i\no = o\no = u\n";
+        parseAndCheckErrors(input, [
+            new parserFacade.Error(4, 4, 5, 6, "undeclared symbol")
+        ]);
+    });
+    describe('should report undeclared symbol for output', function () {
+        let input = "output o\n";
+        parseAndCheckErrors(input, [
+            new parserFacade.Error(1, 1, 8, 9, "undeclared symbol")
+        ]);
+    });
+    describe('should report undeclared symbol for second output', function () {
+        let input = "input i\no = i\noutput o\noutput x\n";
+        parseAndCheckErrors(input, [
+            new parserFacade.Error(4, 4, 8, 9, "undeclared symbol")
+        ]);
+    });
+});
+
+describe('Semantic validation of examples being edited', function () {
+    describe('deleting input identifier', function () {
+        let input = "input a\ninput\ninput a\n";
+        parseAndCheckErrors(input, [
+            new parserFacade.Error(2, 2, 5, 6, "missing ID at '\\n'"),
+            new parserFacade.Error(3, 3, 7, 8, "input already declared")
+        ]);
+    });
+    describe('deleting calculation value still declares the target', function () {
+        let input = "a = 1 + 1\nb = \nc = a + b\n";
+        parseAndCheckErrors(input, [
+            new parserFacade.Error(2, 2, 4, 5, "mismatched input '\\n' expecting {NUMBER_LIT, ID, '(', '-'}"),
+        ]);
+    });
+    describe('deleting output identifier', function () {
+        let input = "input a\noutput\noutput a\n";
+        parseAndCheckErrors(input, [
+            new parserFacade.Error(2, 2, 6, 7, "missing ID at '\\n'"),
+        ]);
+    });
+});