diff --git a/doc/function/assert.txt b/doc/function/assert.txt
new file mode 100644
index 00000000..0a7cafe4
--- /dev/null
+++ b/doc/function/assert.txt
@@ -0,0 +1,16 @@
+Function: assert
+
+--Usage--
+> assert(condition)
+
+Check that the condition is @true@. Shows a warning message if it is not.
+
+Note: @assert@ is a special built-in keyword, so that the error message can include the condition.
+
+--Parameters--
+! Parameter	Type			Description
+| @input@	[[type:boolean]]	Condition to check.
+
+--Examples--
+> assert(1 + 1 == 2) == nil  # nothing happens
+> assert(1 * 1 == 2) == nil  # An error message is shown
diff --git a/doc/function/index.txt b/doc/function/index.txt
index dafb0ecc..1c81b8f7 100644
--- a/doc/function/index.txt
+++ b/doc/function/index.txt
@@ -87,3 +87,4 @@ These functions are built into the program, other [[type:function]]s can be defi
 	
 ! Other functions		<<<
 | [[fun:trace]]			Output a message for debugging purposes.
+| [[fun:assert]]		Check a condition for debugging purposes.
diff --git a/src/script/context.cpp b/src/script/context.cpp
index bac67cbf..82f3b121 100644
--- a/src/script/context.cpp
+++ b/src/script/context.cpp
@@ -201,7 +201,14 @@ void Context::setVariable(const String& name, const ScriptValueP& value) {
 	setVariable(string_to_variable(name), value);
 }
 
+#ifdef _DEBUG
+	extern vector<String> variable_names;
+#endif
+
 void Context::setVariable(Variable name, const ScriptValueP& value) {
+	#ifdef _DEBUG
+		assert((size_t)name < variable_names.size());
+	#endif
 	VariableValue& var = variables[name];
 	if (var.level < level) {
 		// keep shadow copy
diff --git a/src/script/functions/basic.cpp b/src/script/functions/basic.cpp
index 6608a76d..b2763bba 100644
--- a/src/script/functions/basic.cpp
+++ b/src/script/functions/basic.cpp
@@ -31,6 +31,12 @@ SCRIPT_FUNCTION(trace) {
 	SCRIPT_RETURN(input);
 }
 
+SCRIPT_FUNCTION(warning) {
+	SCRIPT_PARAM_C(String, input);
+	handle_warning(input, true);
+	return script_nil;
+}
+
 // ----------------------------------------------------------------------------- : String stuff
 
 // convert a string to upper case
diff --git a/src/script/parser.cpp b/src/script/parser.cpp
index 65802bde..68fd9c6f 100644
--- a/src/script/parser.cpp
+++ b/src/script/parser.cpp
@@ -22,6 +22,8 @@ DECLARE_TYPEOF_COLLECTION(Variable);
 
 String read_utf8_line(wxInputStream& input, bool eat_bom = true, bool until_eof = false);
 
+extern ScriptValueP script_warning;
+
 // ----------------------------------------------------------------------------- : Tokenizing : class
 
 enum TokenType
@@ -75,6 +77,11 @@ class TokenIterator {
 	 */
 	void putBack();
 	
+	/// Get a section of source code
+	String getSourceCode(size_t start, size_t end);
+	/// Get the current line number
+	int getLineNumber();
+	
   private:
 	String input;
 	size_t pos;
@@ -318,6 +325,15 @@ void TokenIterator::expected(const String& expected, const Token* opening) {
 }
 
 
+String TokenIterator::getSourceCode(size_t start, size_t end) {
+	start = min(start, input.size());
+	end   = min(end,   input.size());
+	return input.substr(start, end-start);
+}
+int TokenIterator::getLineNumber() {
+	return line_number(peek(0).pos, input);
+}
+
 // ----------------------------------------------------------------------------- : Parsing
 
 
@@ -450,24 +466,20 @@ void parseExpr(TokenIterator& input, Script& script, Precedence minPrec) {
 				script.addInstruction(I_PUSH_CONST, script_nil); // universal constant : nil
 			} else if (token == _("if")) {
 				// if AAA then BBB else CCC
-				unsigned int jmpElse, jmpEnd;
-				parseOper(input, script, PREC_AND);						// AAA
-				jmpElse = script.getLabel();							//		jmp_else:
-				script.addInstruction(I_JUMP_IF_NOT, INVALID_ADDRESS);	//		jnz lbl_else
-				expectToken(input, _("then"));							// then
-				parseOper(input, script, PREC_SET);						// BBB
-				jmpEnd = script.getLabel();								//		jmp_end:
-				script.addInstruction(I_JUMP, INVALID_ADDRESS);			//		jump lbl_end
-				script.comeFrom(jmpElse);								//		lbl_else:
-				if (input.peek() == _("else")) {						// else
+				parseOper(input, script, PREC_AND);							// AAA
+				unsigned jmpElse = script.addInstruction(I_JUMP_IF_NOT);	//		jnz lbl_else
+				expectToken(input, _("then"));								// then
+				parseOper(input, script, PREC_SET);							// BBB
+				unsigned jmpEnd = script.addInstruction(I_JUMP);			//		jump lbl_end
+				script.comeFrom(jmpElse);									//		lbl_else:
+				if (input.peek() == _("else")) {							//else
 					input.read();
-					parseOper(input, script, PREC_SET);					// CCC
+					parseOper(input, script, PREC_SET);						// CCC
 				} else {
 					script.addInstruction(I_PUSH_CONST, script_nil);
 				}
-				script.comeFrom(jmpEnd);								//		lbl_end:
+				script.comeFrom(jmpEnd);									//		lbl_end:
 			} else if (token == _("for")) {
-				unsigned int lblStart;
 				// the loop body should have a net stack effect of 0, but the entire expression of +1
 				// solution: add all results from the body, start with nil
 				if (input.peek() == _("each")) {
@@ -481,8 +493,7 @@ void parseExpr(TokenIterator& input, Script& script, Precedence minPrec) {
 					parseOper(input, script, PREC_AND);					// BBB
 					script.addInstruction(I_UNARY, I_ITERATOR_C);		//		iterator_collection
 					script.addInstruction(I_PUSH_CONST, script_nil);	//		push nil
-					lblStart = script.getLabel();						//		lbl_start:
-					script.addInstruction(I_LOOP, INVALID_ADDRESS);		//		loop
+					unsigned lblStart = script.addInstruction(I_LOOP);	//		lbl_start: loop lbl_end
 					expectToken(input, _("do"));						// do
 					script.addInstruction(I_SET_VAR,
 										string_to_variable(name.value));//		set name
@@ -499,8 +510,7 @@ void parseExpr(TokenIterator& input, Script& script, Precedence minPrec) {
 					parseOper(input, script, PREC_AND);					// CCC
 					script.addInstruction(I_BINARY, I_ITERATOR_R);		//		iterator_range
 					script.addInstruction(I_PUSH_CONST, script_nil);	//		push nil
-					lblStart = script.getLabel();						//		lbl_start:
-					script.addInstruction(I_LOOP, INVALID_ADDRESS);		//		loop
+					unsigned lblStart = script.addInstruction(I_LOOP);	//		lbl_start: loop lbl_end
 					expectToken(input, _("do"));						// do
 					script.addInstruction(I_SET_VAR,
 										string_to_variable(name.value));//		set name
@@ -542,6 +552,25 @@ void parseExpr(TokenIterator& input, Script& script, Precedence minPrec) {
 					script.addInstruction(I_BINARY, op);
 				}
 				expectToken(input, _(")"), &token);
+			} else if (token == _("assert")) {
+				// assert(condition)
+				expectToken(input, _("("));
+				size_t start = input.peek().pos;
+				int line = input.getLineNumber();
+				parseOper(input, script, PREC_ALL); // condition
+				size_t end = input.peek().pos;
+				String message = String::Format(_("Assertion failure on line %d: "), line) + input.getSourceCode(start,end);
+				expectToken(input, _(")"), &token);
+				// compile into:  if condition then nil else warning("condition")
+				unsigned jmpElse = script.addInstruction(I_JUMP_IF_NOT);	// jnz lbl_else
+				script.addInstruction(I_PUSH_CONST, script_nil);			// push nil
+				unsigned jmpEnd = script.addInstruction(I_JUMP);			// jump lbl_end
+				script.comeFrom(jmpElse);									// lbl_else:
+				script.addInstruction(I_PUSH_CONST, script_warning);		// push warning
+				script.addInstruction(I_PUSH_CONST, message);				// push "condition"
+				script.addInstruction(I_CALL, 1);							// call
+				script.addInstruction(I_NOP,  SCRIPT_VAR_input);			// (input:)
+				script.comeFrom(jmpEnd);									// lbl_end:
 			} else {
 				// variable
 				Variable var = string_to_variable(token.value);
diff --git a/src/script/script.cpp b/src/script/script.cpp
index 678d6ee8..3f012d80 100644
--- a/src/script/script.cpp
+++ b/src/script/script.cpp
@@ -86,10 +86,14 @@ ScriptValueP Script::eval(Context& ctx) const {
 ScriptValueP Script::dependencies(Context& ctx, const Dependency& dep) const {
 	return ctx.dependencies(dep, *this);
 }
-	
-void Script::addInstruction(InstructionType t) {
-	Instruction i = {t, {0}};
+
+static const unsigned int INVALID_ADDRESS = 0x03FFFFFF;
+
+unsigned int Script::addInstruction(InstructionType t) {
+	assert( t == I_JUMP || t == I_JUMP_IF_NOT || t == I_LOOP);
+	Instruction i = {t, {INVALID_ADDRESS}};
 	instructions.push_back(i);
+	return getLabel() - 1;
 }
 void Script::addInstruction(InstructionType t, unsigned int d) {
 	// Don't optimize ...I_PUSH_CONST x; I_MEMBER... to I_MEMBER_C
@@ -198,8 +202,7 @@ String Script::dumpInstr(unsigned int pos, Instruction i) const {
 	// arg
 	switch (i.instr) {
 		case I_PUSH_CONST: case I_MEMBER_C:							// const
-			ret += _("\t") + constants[i.data]->toString();
-			ret += _("\t(") + constants[i.data]->typeName() + _(")");
+			ret += _("\t") + constants[i.data]->typeName();
 			break;
 		case I_JUMP: case I_JUMP_IF_NOT: case I_LOOP: case I_MAKE_OBJECT: case I_CALL: case I_CLOSURE:	// int
 			ret += String::Format(_("\t%d"), i.data);
diff --git a/src/script/script.hpp b/src/script/script.hpp
index 043aa121..b8c33283 100644
--- a/src/script/script.hpp
+++ b/src/script/script.hpp
@@ -103,7 +103,6 @@ struct Instruction {
 		QuaternaryInstructionType	instr4 : 27;
 	};
 };
-static const unsigned int INVALID_ADDRESS = 0x03FFFFFF;
 
 // ----------------------------------------------------------------------------- : Variables
 
@@ -157,8 +156,8 @@ class Script : public ScriptValue {
 	virtual ScriptValueP eval(Context& ctx) const;
 	virtual ScriptValueP dependencies(Context& ctx, const Dependency&) const;
 	
-	/// Add an instruction with no data
-	void addInstruction(InstructionType t);
+	/// Add a jump instruction, later comeFrom should be called on the returned value
+	unsigned int addInstruction(InstructionType t);
 	/// Add an instruction with integer data
 	void addInstruction(InstructionType t, unsigned int d);
 	/// Add an instruction with constant data
@@ -169,7 +168,6 @@ class Script : public ScriptValue {
 	/// Update an instruction to point to the current position
 	/** The instruction at pos must be a jumping instruction, it is changed so the current position
 	 *  'comes from' pos (in addition to other control flow).
-	 *  The position must be a label just before the instruction to be updated.
 	 */
 	void comeFrom(unsigned int pos);
 	/// Get the current instruction position
diff --git a/tools/website/drupal/mse-drupal-modules/highlight.inc b/tools/website/drupal/mse-drupal-modules/highlight.inc
index 4988e511..511a8455 100644
--- a/tools/website/drupal/mse-drupal-modules/highlight.inc
+++ b/tools/website/drupal/mse-drupal-modules/highlight.inc
@@ -69,7 +69,9 @@ $built_in_functions = array(
 	'copy_file'		=>'',
 	'write_text_file'	=>'',
 	'write_image_file'	=>'',
+	// other
 	'trace'			=>'',
+	'assert'		=>'',
 );