diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 48f1c58..e206ea5 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -3,6 +3,57 @@ name: CI
 on: [push, pull_request]
 
 jobs:
+  blackbox:
+    runs-on: ${{ matrix.os }}
+    strategy:
+      matrix:
+        os: [ubuntu-latest, macOS-latest]
+        php-version: ['8.2', '8.3']
+        dependency-versions: ['lowest', 'highest']
+    name: 'BlackBox'
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v2
+      - name: Setup PHP
+        uses: shivammathur/setup-php@v2
+        with:
+          php-version: ${{ matrix.php-version }}
+          extensions: mbstring, intl
+          coverage: none
+      - name: Composer
+        uses: "ramsey/composer-install@v2"
+        with:
+          dependency-versions: ${{ matrix.dependencies }}
+      - name: BlackBox
+        run: php blackbox.php
+  coverage:
+    runs-on: ${{ matrix.os }}
+    strategy:
+      matrix:
+        os: [ubuntu-latest, macOS-latest]
+        php-version: ['8.2', '8.3']
+        dependency-versions: ['lowest', 'highest']
+    name: 'Coverage'
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v2
+      - name: Setup PHP
+        uses: shivammathur/setup-php@v2
+        with:
+          php-version: ${{ matrix.php-version }}
+          extensions: mbstring, intl
+          coverage: xdebug
+      - name: Composer
+        uses: "ramsey/composer-install@v2"
+        with:
+          dependency-versions: ${{ matrix.dependencies }}
+      - name: BlackBox
+        run: php blackbox.php
+        env:
+          ENABLE_COVERAGE: 'true'
+      - uses: codecov/codecov-action@v1
+        with:
+          token: ${{ secrets.CODECOV_TOKEN }}
   psalm:
     runs-on: ubuntu-latest
     strategy:
diff --git a/.php-cs-fixer.dist.php b/.php-cs-fixer.dist.php
index 1431d66..021fd34 100644
--- a/.php-cs-fixer.dist.php
+++ b/.php-cs-fixer.dist.php
@@ -1,6 +1,6 @@
 <?php
 
 return Innmind\CodingStandard\CodingStandard::config([
-    'tests',
+    'proofs',
     'src',
 ]);
diff --git a/blackbox.php b/blackbox.php
new file mode 100644
index 0000000..3785ad1
--- /dev/null
+++ b/blackbox.php
@@ -0,0 +1,27 @@
+<?php
+declare(strict_types = 1);
+
+require 'vendor/autoload.php';
+
+use Innmind\BlackBox\{
+    Application,
+    Runner\Load,
+    Runner\CodeCoverage,
+};
+
+Application::new($argv)
+    ->when(
+        \getenv('ENABLE_COVERAGE') !== false,
+        static fn(Application $app) => $app
+            ->codeCoverage(
+                CodeCoverage::of(
+                    __DIR__.'/src/',
+                    __DIR__.'/proofs/',
+                )
+                    ->dumpTo('coverage.clover')
+                    ->enableWhen(true),
+            )
+            ->scenariiPerProof(1),
+    )
+    ->tryToProve(Load::everythingIn(__DIR__.'/proofs/'))
+    ->exit();
diff --git a/proofs/functional.php b/proofs/functional.php
new file mode 100644
index 0000000..759270e
--- /dev/null
+++ b/proofs/functional.php
@@ -0,0 +1,179 @@
+<?php
+declare(strict_types = 1);
+
+use Innmind\Mantle\{
+    Forerunner,
+    Source\Predetermined,
+    Task,
+};
+use Innmind\OperatingSystem\Factory;
+use Innmind\TimeContinuum\Earth\Period\Second;
+use Innmind\Filesystem\Name;
+use Innmind\Url\Path;
+use Innmind\Immutable\Sequence;
+use Innmind\BlackBox\Set;
+use Innmind\BlackBox\Tag;
+
+return static function() {
+    yield test(
+        'Halting multiple tasks',
+        static function($assert) {
+            $expect = $assert->time(static function() {
+                Forerunner::of(Factory::build())(null, Predetermined::of(
+                    static fn($os) => $os->process()->halt(Second::of(1)),
+                    static fn($os) => $os->process()->halt(Second::of(1)),
+                    static fn($os) => $os->process()->halt(Second::of(1)),
+                ));
+            });
+            $expect
+                ->inLessThan()
+                ->seconds(2);
+            $expect
+                ->inMoreThan()
+                ->seconds(1);
+        },
+    );
+
+    yield proof(
+        'Carry value via the source',
+        given(
+            Set\Type::any(),
+            Set\Type::any(),
+        ),
+        static function($assert, $initial, $modified) {
+            $returned = Forerunner::of(Factory::build())(
+                $initial,
+                static fn($_, $__, $continuation) => $continuation->terminate(),
+            );
+            $assert->same($initial, $returned);
+
+            $returned = Forerunner::of(Factory::build())(
+                $initial,
+                static fn($carry, $__, $continuation) => $continuation
+                    ->carryWith($initial)
+                    ->terminate(),
+            );
+            $assert->same($initial, $returned);
+
+            $returned = Forerunner::of(Factory::build())(
+                $initial,
+                static fn($carry, $__, $continuation) => $continuation
+                    ->carryWith($modified)
+                    ->terminate(),
+            );
+            $assert->same($modified, $returned);
+        },
+    );
+
+    yield test(
+        'The source is run asynchronously',
+        static function($assert) {
+            $expect = $assert->time(static function() {
+                Forerunner::of(Factory::build())(
+                    false,
+                    static fn($started, $os, $continuation, $results) => match ([$started, $results->size()]) {
+                        [false, 0] => $continuation
+                            ->launch(Sequence::of(
+                                Task::of(static function($os) {
+                                    $os->process()->halt(Second::of(1));
+                                    $os->process()->halt(Second::of(1));
+                                }),
+                                Task::of(static function($os) {
+                                    $os->process()->halt(Second::of(1));
+                                    $os->process()->halt(Second::of(1));
+                                }),
+                                Task::of(static function($os) {
+                                    $os->process()->halt(Second::of(1));
+                                    $os->process()->halt(Second::of(1));
+                                }),
+                            ))
+                            ->carryWith(true),
+                        [true, 0] => (static function($os, $continuation) {
+                            // this halt is executed at the same time at the
+                            // second one in each task
+                            $os->process()->halt(Second::of(1));
+
+                            return $continuation;
+                        })($os, $continuation),
+                        default => $continuation->terminate(),
+                    },
+                );
+            });
+            $expect
+                ->inLessThan()
+                ->seconds(3);
+            $expect
+                ->inMoreThan()
+                ->seconds(2);
+        },
+    );
+
+    yield test(
+        'Streams are handled asynchronously',
+        static function($assert) {
+            $lines = [];
+            Forerunner::of(Factory::build())(null, Predetermined::of(
+                static function($os) use ($assert, &$lines) {
+                    $file = $os
+                        ->filesystem()
+                        ->mount(Path::of('./'))
+                        ->get(Name::of('composer.json'))
+                        ->match(
+                            static fn($file) => $file,
+                            static fn() => null,
+                        );
+                    $assert->not()->null($file);
+                    $lines[] = $file
+                        ->content()
+                        ->lines()
+                        ->first()
+                        ->match(
+                            static fn($line) => $line->toString(),
+                            static fn() => null,
+                        );
+                    $lines[] = $file
+                        ->content()
+                        ->lines()
+                        ->filter(static fn($line) => !$line->str()->empty())
+                        ->last()
+                        ->match(
+                            static fn($line) => $line->toString(),
+                            static fn() => null,
+                        );
+                },
+                static function($os) use ($assert, &$lines) {
+                    $file = $os
+                        ->filesystem()
+                        ->mount(Path::of('./'))
+                        ->get(Name::of('LICENSE'))
+                        ->match(
+                            static fn($file) => $file,
+                            static fn() => null,
+                        );
+                    $assert->not()->null($file);
+                    $lines[] = $file
+                        ->content()
+                        ->lines()
+                        ->first()
+                        ->match(
+                            static fn($line) => $line->toString(),
+                            static fn() => null,
+                        );
+                    $lines[] = $file
+                        ->content()
+                        ->lines()
+                        ->filter(static fn($line) => !$line->str()->empty())
+                        ->last()
+                        ->match(
+                            static fn($line) => $line->toString(),
+                            static fn() => null,
+                        );
+                },
+            ));
+            $assert->same(
+                ['{', 'MIT License', 'SOFTWARE.', '}'],
+                $lines,
+            );
+        },
+    )->tag(Tag::wip);
+};
diff --git a/src/Source/Context.php b/src/Source/Context.php
index 42d4e5d..329de47 100644
--- a/src/Source/Context.php
+++ b/src/Source/Context.php
@@ -70,12 +70,32 @@ public static function of(Source $source, mixed $carry): self
 
     /**
      * @param Sequence<R> $results
+     *
+     * @return self<C, R>
      */
     public function withResults(Sequence $results): self
     {
         return new self($this->source, $this->continuation, $results);
     }
 
+    /**
+     * @template R1
+     * @template R2
+     *
+     * @param callable(Source<C, R>, C): R1 $resume
+     * @param callable(C): R2 $terminate
+     *
+     * @return R1|R2
+     */
+    public function match(callable $resume, callable $terminate): mixed
+    {
+        /** @psalm-suppress MixedArgument */
+        return $this->continuation->match(
+            fn($carry) => $resume($this->source, $carry),
+            static fn($carry) => $terminate($carry),
+        );
+    }
+
     /**
      * @return Continuation<C, R>
      */
diff --git a/src/Tasks.php b/src/Tasks.php
index 55db98a..4cc85e1 100644
--- a/src/Tasks.php
+++ b/src/Tasks.php
@@ -112,9 +112,8 @@ public function continue(OperatingSystem $os): self
         $source = $source->flatMap(static fn($task) => match (true) {
             $task instanceof Task\Terminated && $task->returned() instanceof Context => $task
                 ->returned()
-                ->continuation()
                 ->match(
-                    static fn() => Either::right($task->returned()),
+                    static fn($source, $carry) => Either::right(Context::of($source, $carry)),
                     static fn($carry) => Either::left($carry),
                 ),
             default => Either::right($task),
diff --git a/src/Wait.php b/src/Wait.php
index 53344f5..7955588 100644
--- a/src/Wait.php
+++ b/src/Wait.php
@@ -116,12 +116,15 @@ public function __invoke(): array
             static fn() => null,
         );
         /** @psalm-suppress InvalidArgument */
-        $watch = $this
-            ->os
-            ->sockets()
-            ->watch($timeout)
-            ->forRead(...$forRead->toList())
-            ->forWrite(...$forWrite->toList());
+        $watch = $this->os->sockets()->watch($timeout);
+        $watch = $forRead->sort(fn($a, $b) => 0)->match(
+            static fn($read, $rest) => $watch->forRead($read, ...$rest->toList()),
+            static fn() => $watch,
+        );
+        $watch = $forWrite->sort(fn($a, $b) => 0)->match(
+            static fn($write, $rest) => $watch->forWrite($write, ...$rest->toList()),
+            static fn() => $watch,
+        );
 
         $ready = $watch();
         $took = $this->os->clock()->now()->elapsedSince($started);