diff --git a/CHANGELOG.md b/CHANGELOG.md
index 3ffb620..50d57a6 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -7,9 +7,19 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+### Added
+
+- new builtins: map, while, set_len, duration
+
 ### Changed
 
 - Block can be called with more parameters then it requires
+- Removed `[]` sequence notation in favour of `()`
+- reorganized fold argument order
+
+### Fixed
+
+- Index operation using booleans behaves like a mask and not fancy way of spelling 0 and 1
 
 ## [0.2.1] - 2022-10-21
 
diff --git a/examples/control-flow.mq b/examples/control-flow.mq
new file mode 100644
index 0000000..019a633
--- /dev/null
+++ b/examples/control-flow.mq
@@ -0,0 +1,13 @@
+
+say (if true 4 2);
+say (if false 4 2);
+
+if true  (say 4) (say 2);
+if false (say 4) (say 2);
+
+n := 0;
+while (n < 5) (
+	say 'while n;
+	n = n + 1;
+);
+
diff --git a/examples/factorial.mq b/examples/factorial.mq
index a3af654..a684b0a 100644
--- a/examples/factorial.mq
+++ b/examples/factorial.mq
@@ -19,7 +19,7 @@ factorial_iterative := (n |
 );
 
 -- Calculate factorial using composition of functions
-factorial := (n | fold (1 + up n) 1 '*);
+factorial := (n | fold '* 1 (1 + up n));
 
 -- Gather all functions into array, and iterate over it
 -- This allows to reduce repeatition of this test case
diff --git a/examples/noises.mq b/examples/noises.mq
new file mode 100644
index 0000000..d664152
--- /dev/null
+++ b/examples/noises.mq
@@ -0,0 +1,25 @@
+-- todo: cicho, ostatni parametr midi
+-- todo: for doesn't forwards implicit play context
+
+pick := (array | (shuffle array).0);
+
+hand1_pool := (
+	chord (g# 3) (g 4);
+	chord (d# 3)  (d 4);
+	chord (g 3) (g# 4);
+	chord (d 3) (d# 4)
+);
+
+hand2_pool := (d 8; d# 8; g 8; g# 8; d 9; d# 9);
+
+for (up 10) (
+	hand1_length := pick (hn; dhn);
+	hand1 := (set_len hand1_length (pick hand1_pool));
+
+	hand2 := ();
+	while (fold '+ (map duration hand2) != hand1_length) (
+		hand2 = flat hand2 (set_len (1/64) (pick hand2_pool));
+	);
+
+	sim hand1 hand2;
+);
diff --git a/examples/permutations.mq b/examples/permutations.mq
index 8873cd1..8a6c2c4 100644
--- a/examples/permutations.mq
+++ b/examples/permutations.mq
@@ -1,4 +1,4 @@
-factorial := (n | fold (1 + up n) '*);
+factorial := (n | fold '* (1 + up n));
 
 
 list_all_permutations := ( array |
diff --git a/musique/interpreter/builtin_functions.cc b/musique/interpreter/builtin_functions.cc
index 9329b8f..ab21e28 100644
--- a/musique/interpreter/builtin_functions.cc
+++ b/musique/interpreter/builtin_functions.cc
@@ -525,35 +525,57 @@ static Result<Value> builtin_fold(Interpreter &interpreter, std::vector<Value> a
 	constexpr auto guard = Guard<2> {
 		.name = "fold",
 		.possibilities = {
-			"(array, callback) -> any",
-			"(array, init, callback) -> any"
+			"(callback, ...values) -> any",
 		}
 	};
 
-	Value array, init, callback;
-	switch (args.size()) {
-	break; case 2:
-		array    = std::move(args[0]);
-		callback = std::move(args[1]);
-		if (array.size() != 0) {
-			init = Try(array.index(interpreter, 0));
+	if (args.size()) {
+		if (auto p = get_if<Function>(args.front())) {
+			auto xs = Try(flatten(interpreter, std::span(args).subspan(1)));
+			if (xs.empty()) {
+				return Value{};
+			}
+			auto init = xs[0];
+			for (auto i = 1u; i < xs.size(); ++i) {
+				init = Try((*p)(interpreter, { std::move(init), std::move(xs[i]) }));
+			}
+			return init;
 		}
-	break; case 3:
-		array    = std::move(args[0]);
-		init     = std::move(args[1]);
-		callback = std::move(args[2]);
-	break; default:
+	}
+
+	return guard.yield_error();
+}
+
+static Result<Value> builtin_map(Interpreter &interpreter, std::vector<Value> args)
+{
+	static constexpr auto guard = Guard<2> {
+		.name = "fold",
+		.possibilities = {
+			"(callback, array) -> any",
+			"(callback, array, init) -> any"
+		}
+	};
+
+	if (args.empty()) {
 		return guard.yield_error();
 	}
 
-	auto collection = Try(guard.match<Collection>(array));
-	auto function = Try(guard.match<Function>(callback));
+	auto function = Try(guard.match<Function>(args.front()));
 
-	for (auto i = 0u; i < collection->size(); ++i) {
-		auto element = Try(collection->index(interpreter, i));
-		init = Try((*function)(interpreter, { std::move(init), std::move(element) }));
+	std::vector<Value> result;
+
+	for (auto &arg : std::span(args).subspan(1)) {
+		if (auto collection = get_if<Collection>(arg)) {
+			for (auto i = 0u; i < collection->size(); ++i) {
+				auto element = Try(collection->index(interpreter, i));
+				result.push_back(Try((*function)(interpreter, { std::move(element) })));
+			}
+		} else {
+			result.push_back(Try((*function)(interpreter, { std::move(arg) })));
+		}
 	}
-	return init;
+
+	return result;
 }
 
 /// Scan computes inclusive prefix sum
@@ -601,6 +623,29 @@ static Result<Value> builtin_if(Interpreter &i, std::span<Ast> args)  {
 	return Value{};
 }
 
+/// Loop block depending on condition
+static Result<Value> builtin_while(Interpreter &i, std::span<Ast> args)  {
+	static constexpr auto guard = Guard<2> {
+		.name = "while",
+		.possibilities = {
+			"(any, function) -> any"
+		}
+	};
+
+	if (args.size() != 2) {
+		return guard.yield_error();
+	}
+
+	while (Try(i.eval((Ast)args.front())).truthy()) {
+		if (args[1].type == Ast::Type::Block) {
+			Try(i.eval((Ast)args[1].arguments.front()));
+		} else {
+			Try(i.eval((Ast)args[1]));
+		}
+	}
+	return Value{};
+}
+
 /// Try executing all but last block and if it fails execute last one
 static Result<Value> builtin_try(Interpreter &interpreter, std::vector<Value> args)
 {
@@ -680,6 +725,65 @@ static Result<Value> builtin_len(Interpreter &i, std::vector<Value> args)
 	return ctx_read_write_property<&Context::length>(i, std::move(args));
 }
 
+Result<Value> traverse(Interpreter &interpreter, Value &&value, auto &&lambda)
+{
+	if (auto collection = get_if<Collection>(value)) {
+		std::vector<Value> flat;
+		for (auto i = 0u; i < collection->size(); ++i) {
+			Value v = Try(collection->index(interpreter, i));
+			flat.push_back(Try(traverse(interpreter, std::move(v), lambda)));
+		}
+		return flat;
+	}
+
+	std::visit([&lambda]<typename T>(T &value) {
+		if constexpr (requires { {lambda(value)}; }) {
+			lambda(value);
+		}
+	}, value.data);
+	return value;
+}
+
+/// Set length (first argument) to all other arguments preserving their shape
+static Result<Value> builtin_set_len(Interpreter &interpreter, std::vector<Value> args)
+{
+	if (auto len = get_if<Number>(args.front())) {
+		std::vector<Value> result;
+		for (auto &arg : std::span(args).subspan(1)) {
+			auto arg_result = Try(traverse(interpreter, std::move(arg), [&](Chord &c) {
+				for (Note &note : c.notes) {
+					note.length = *len;
+				}
+			}));
+			result.push_back(std::move(arg_result));
+		}
+		return result;
+	}
+
+	return errors::Unsupported_Types_For {
+		.type = errors::Unsupported_Types_For::Function,
+		.name = "set_len",
+		.possibilities = {
+			"TODO"
+		}
+	};
+}
+
+static Result<Value> builtin_duration(Interpreter &interpreter, std::vector<Value> args)
+{
+	auto total = Number{};
+	for (auto &arg : args) {
+		Try(traverse(interpreter, std::move(arg), [&](Chord &c) {
+			for (Note &note : c.notes) {
+				if (note.length) {
+					total += *note.length;
+				}
+			}
+		}));
+	}
+	return total;
+}
+
 /// Join arguments into flat array
 static Result<Value> builtin_flat(Interpreter &i, std::vector<Value> args)
 {
@@ -961,6 +1065,7 @@ void Interpreter::register_builtin_functions()
 	global.force_define("ceil",           apply_numeric_transform<&Number::ceil>);
 	global.force_define("chord",          builtin_chord);
 	global.force_define("down",           builtin_range<Range_Direction::Down>);
+	global.force_define("duration",       builtin_duration);
 	global.force_define("flat",           builtin_flat);
 	global.force_define("floor",          apply_numeric_transform<&Number::floor>);
 	global.force_define("fold",           builtin_fold);
@@ -969,6 +1074,7 @@ void Interpreter::register_builtin_functions()
 	global.force_define("if",             builtin_if);
 	global.force_define("instrument",     builtin_program_change);
 	global.force_define("len",            builtin_len);
+	global.force_define("map",            builtin_map);
 	global.force_define("max",            builtin_max);
 	global.force_define("min",            builtin_min);
 	global.force_define("mix",            builtin_mix);
@@ -987,6 +1093,7 @@ void Interpreter::register_builtin_functions()
 	global.force_define("rotate",         builtin_rotate);
 	global.force_define("round",          apply_numeric_transform<&Number::round>);
 	global.force_define("scan",           builtin_scan);
+	global.force_define("set_len",        builtin_set_len);
 	global.force_define("shuffle",        builtin_shuffle);
 	global.force_define("sim",            builtin_sim);
 	global.force_define("sort",           builtin_sort);
@@ -996,4 +1103,5 @@ void Interpreter::register_builtin_functions()
 	global.force_define("unique",         builtin_unique);
 	global.force_define("up",             builtin_range<Range_Direction::Up>);
 	global.force_define("update",         builtin_update);
+	global.force_define("while",          builtin_while);
 }
diff --git a/musique/main.cc b/musique/main.cc
index 234b88b..813464e 100644
--- a/musique/main.cc
+++ b/musique/main.cc
@@ -190,7 +190,7 @@ struct Runner
 				if (std::next(it) != args.end())
 					std::cout << ' ';
 			}
-			std::cout << '\n';
+			std::cout << std::endl;
 			return {};
 		});
 	}