diff --git a/Exception/NoResult.php b/Exception/NoResult.php
new file mode 100644
index 0000000..6d411f3
--- /dev/null
+++ b/Exception/NoResult.php
@@ -0,0 +1,6 @@
+<?php
+namespace Hoa\Ruler\Exception;
+
+class NoResult extends Exception
+{
+}
diff --git a/Exception/RuleDoesNotValidate.php b/Exception/RuleDoesNotValidate.php
new file mode 100644
index 0000000..128adf9
--- /dev/null
+++ b/Exception/RuleDoesNotValidate.php
@@ -0,0 +1,6 @@
+<?php
+namespace Hoa\Ruler\Exception;
+
+class RuleDoesNotValidate extends Exception
+{
+}
diff --git a/Result.php b/Result.php
new file mode 100644
index 0000000..4245d3c
--- /dev/null
+++ b/Result.php
@@ -0,0 +1,105 @@
+<?php
+
+/**
+ * Hoa
+ *
+ *
+ * @license
+ *
+ * New BSD License
+ *
+ * Copyright © 2007-2016, Hoa community. All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are met:
+ *     * Redistributions of source code must retain the above copyright
+ *       notice, this list of conditions and the following disclaimer.
+ *     * Redistributions in binary form must reproduce the above copyright
+ *       notice, this list of conditions and the following disclaimer in the
+ *       documentation and/or other materials provided with the distribution.
+ *     * Neither the name of the Hoa nor the names of its contributors may be
+ *       used to endorse or promote products derived from this software without
+ *       specific prior written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+ * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+ * ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDERS AND CONTRIBUTORS BE
+ * LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
+ * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
+ * SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
+ * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
+ * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
+ * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
+ * POSSIBILITY OF SUCH DAMAGE.
+ */
+
+namespace Hoa\Ruler;
+
+
+/**
+ * @property string name
+ * @property Rule   rule
+ * @property mixed  $result
+ */
+class Result
+{
+    /**
+     * @var string
+     */
+    private $name;
+
+    /**
+     * @var Rule
+     */
+    private $rule;
+
+    /**
+     * @var mixed
+     */
+    private $result;
+
+    /**
+     * @param string $name
+     * @param Rule   $rule
+     * @param mixed  $result
+     */
+    public function __construct($name, Rule $rule, $result)
+    {
+        $this->name   = $name;
+        $this->rule   = $rule;
+        $this->result = $result;
+    }
+
+    /**
+     * @param string $name
+     *
+     * @throws \Error
+     *
+     * @return mixed
+     */
+    public function __get($name)
+    {
+        switch (strtolower($name)) {
+            case 'name':
+                return $name;
+
+            case 'result':
+                if (is_object($this->result)) {
+                    if ($this->result instanceof \Closure) {
+                        return $this->result;
+                    }
+
+                    return clone $this->result;
+                }
+
+                return $this->result;
+
+            case 'rule':
+                return clone $this->rule;
+
+            default:
+                throw new Exception\Exception('Unable to get property ' . $name);
+        }
+    }
+}
diff --git a/Rule.php b/Rule.php
new file mode 100644
index 0000000..611b897
--- /dev/null
+++ b/Rule.php
@@ -0,0 +1,62 @@
+<?php
+
+/**
+ * Hoa
+ *
+ *
+ * @license
+ *
+ * New BSD License
+ *
+ * Copyright © 2007-2016, Hoa community. All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are met:
+ *     * Redistributions of source code must retain the above copyright
+ *       notice, this list of conditions and the following disclaimer.
+ *     * Redistributions in binary form must reproduce the above copyright
+ *       notice, this list of conditions and the following disclaimer in the
+ *       documentation and/or other materials provided with the distribution.
+ *     * Neither the name of the Hoa nor the names of its contributors may be
+ *       used to endorse or promote products derived from this software without
+ *       specific prior written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+ * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+ * ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDERS AND CONTRIBUTORS BE
+ * LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
+ * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
+ * SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
+ * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
+ * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
+ * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
+ * POSSIBILITY OF SUCH DAMAGE.
+ */
+
+namespace Hoa\Ruler;
+
+/**
+ * Interface \Hoa\Ruler\Rule.
+ *
+ * @copyright  Copyright © 2007-2016 Hoa community
+ * @license    New BSD License
+ */
+interface Rule
+{
+    /**
+     * @param Ruler   $ruler
+     * @param Context $context
+     *
+     * @return bool
+     */
+    public function valid(Ruler $ruler, Context $context);
+
+    /**
+     * @param Ruler   $ruler
+     * @param Context $context
+     *
+     * @return mixed
+     */
+    public function execute(Ruler $ruler, Context $context);
+}
diff --git a/Rules.php b/Rules.php
new file mode 100644
index 0000000..dc6f09a
--- /dev/null
+++ b/Rules.php
@@ -0,0 +1,160 @@
+<?php
+
+/**
+ * Hoa
+ *
+ *
+ * @license
+ *
+ * New BSD License
+ *
+ * Copyright © 2007-2016, Hoa community. All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are met:
+ *     * Redistributions of source code must retain the above copyright
+ *       notice, this list of conditions and the following disclaimer.
+ *     * Redistributions in binary form must reproduce the above copyright
+ *       notice, this list of conditions and the following disclaimer in the
+ *       documentation and/or other materials provided with the distribution.
+ *     * Neither the name of the Hoa nor the names of its contributors may be
+ *       used to endorse or promote products derived from this software without
+ *       specific prior written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+ * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+ * ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDERS AND CONTRIBUTORS BE
+ * LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
+ * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
+ * SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
+ * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
+ * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
+ * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
+ * POSSIBILITY OF SUCH DAMAGE.
+ */
+
+namespace Hoa\Ruler;
+
+use Hoa\Heap\Max;
+use Hoa\Ruler\Exception\NoResult;
+use Hoa\Ruler\Exception\RuleDoesNotValidate;
+
+/**
+ * Class \Hoa\Ruler\Rules.
+ *
+ * A collection of rules ordered by priority.
+ *
+ * @copyright  Copyright © 2007-2016 Hoa community
+ * @license    New BSD License
+ */
+class Rules
+{
+    /**
+     * @var Max
+     */
+    private $queue;
+
+    public function __construct()
+    {
+        $this->queue = new Max();
+    }
+
+    /**
+     * @param string $name
+     * @param Rule   $rule
+     * @param int    $priority
+     *
+     * @return Rules
+     */
+    public function add($name, Rule $rule, $priority = -1)
+    {
+        $this->queue->insert([$name, $rule], $priority);
+
+        return $this;
+    }
+
+    /**
+     * @param Ruler   $ruler
+     * @param Context $context
+     *
+     * @throws NoResult
+     *
+     * @return Result
+     */
+    public function getBestResult(Ruler $ruler, Context $context)
+    {
+        $ruler = self::initializeRuler($ruler, $context);
+
+        foreach ($this->queue as $nameAndRule) {
+            /**
+             * @var string $name
+             * @var Rule   $rule
+             */
+            list($name, $rule) = $nameAndRule;
+
+            try {
+                $context['.' . $name] = $rule->execute($ruler, $context);
+            } catch (RuleDoesNotValidate $exception) {
+                $context['.' . $name] = null;
+            }
+
+            if ($rule->valid($ruler, $context)) {
+                return new Result($name, $rule, $context['.' . $name]);
+            }
+        }
+
+        throw new NoResult();
+    }
+
+    /**
+     * @param Ruler   $ruler
+     * @param Context $context
+     *
+     * @return Result[]
+     */
+    public function getAllResults(Ruler $ruler, Context $context)
+    {
+        $ruler   = self::initializeRuler($ruler, $context);
+        $results = [];
+
+        foreach ($this->queue as $nameAndRule) {
+            /**
+             * @var string $name
+             * @var Rule   $rule
+             */
+            list($name, $rule) = $nameAndRule;
+
+            try {
+                $context['.' . $name] = $rule->execute($ruler, $context);
+            } catch (RuleDoesNotValidate $exception) {
+                $context['.' . $name] = null;
+            }
+
+            $results[] = new Result($name, $rule, $context['.' . $name]);
+        }
+
+        return $results;
+    }
+
+    /**
+     * @param Ruler $ruler
+     * @param array $context
+     *
+     * @return Ruler
+     */
+    private static function initializeRuler(Ruler $ruler, Context $context)
+    {
+        if ($ruler->getDefaultAsserter()->operatorExists('rule')) {
+            return $ruler;
+        }
+
+        $ruler = clone $ruler;
+
+        $ruler->getDefaultAsserter()->setOperator('rule', function($id) use ($context) {
+            return $context['.' . $id];
+        });
+
+        return $ruler;
+    }
+}
diff --git a/Rules/Then.php b/Rules/Then.php
new file mode 100644
index 0000000..2dd2b11
--- /dev/null
+++ b/Rules/Then.php
@@ -0,0 +1,100 @@
+<?php
+
+/**
+ * Hoa
+ *
+ *
+ * @license
+ *
+ * New BSD License
+ *
+ * Copyright © 2007-2016, Hoa community. All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are met:
+ *     * Redistributions of source code must retain the above copyright
+ *       notice, this list of conditions and the following disclaimer.
+ *     * Redistributions in binary form must reproduce the above copyright
+ *       notice, this list of conditions and the following disclaimer in the
+ *       documentation and/or other materials provided with the distribution.
+ *     * Neither the name of the Hoa nor the names of its contributors may be
+ *       used to endorse or promote products derived from this software without
+ *       specific prior written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+ * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+ * ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDERS AND CONTRIBUTORS BE
+ * LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
+ * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
+ * SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
+ * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
+ * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
+ * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
+ * POSSIBILITY OF SUCH DAMAGE.
+ */
+
+namespace Hoa\Ruler\Rules;
+
+use Hoa\Ruler\Context;
+use Hoa\Ruler\Exception\RuleDoesNotValidate;
+use Hoa\Ruler\Model\Model;
+use Hoa\Ruler\Rule;
+use Hoa\Ruler\Ruler;
+
+/**
+ * Class \Hoa\Ruler\Rules\Then.
+ *
+ * A rule returning a value when the assertion passes.
+ *
+ * @copyright  Copyright © 2007-2016 Hoa community
+ * @license    New BSD License
+ */
+class Then implements Rule
+{
+    /**
+     * @var string|Model
+     */
+    private $rule;
+
+    /**
+     * @var mixed
+     */
+    private $result;
+
+    /**
+     * @param string $rule
+     * @param mixed  $result
+     */
+    public function __construct($rule, $result)
+    {
+        $this->rule   = $rule;
+        $this->result = $result;
+    }
+
+    /**
+     * @param Ruler   $ruler
+     * @param Context $context
+     *
+     * @return bool
+     */
+    public function valid(Ruler $ruler, Context $context)
+    {
+        return $ruler->assert($this->rule, $context);
+    }
+
+    /**
+     * @param Ruler   $ruler
+     * @param Context $context
+     *
+     * @return mixed
+     */
+    public function execute(Ruler $ruler, Context $context)
+    {
+        if ($this->valid($ruler, $context)) {
+            return $this->result;
+        }
+
+        throw new RuleDoesNotValidate('Rule ' . $this->rule . ' does not validate');
+    }
+}
diff --git a/Rules/ThenElse.php b/Rules/ThenElse.php
new file mode 100644
index 0000000..159347e
--- /dev/null
+++ b/Rules/ThenElse.php
@@ -0,0 +1,101 @@
+<?php
+
+/**
+ * Hoa
+ *
+ *
+ * @license
+ *
+ * New BSD License
+ *
+ * Copyright © 2007-2016, Hoa community. All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are met:
+ *     * Redistributions of source code must retain the above copyright
+ *       notice, this list of conditions and the following disclaimer.
+ *     * Redistributions in binary form must reproduce the above copyright
+ *       notice, this list of conditions and the following disclaimer in the
+ *       documentation and/or other materials provided with the distribution.
+ *     * Neither the name of the Hoa nor the names of its contributors may be
+ *       used to endorse or promote products derived from this software without
+ *       specific prior written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+ * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+ * ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDERS AND CONTRIBUTORS BE
+ * LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
+ * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
+ * SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
+ * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
+ * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
+ * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
+ * POSSIBILITY OF SUCH DAMAGE.
+ */
+
+namespace Hoa\Ruler\Rules;
+
+use Hoa\Ruler\Context;
+use Hoa\Ruler\Exception\RuleDoesNotValidate;
+use Hoa\Ruler\Rule;
+use Hoa\Ruler\Ruler;
+
+/**
+ * Class \Hoa\Ruler\Rules\ThenElse.
+ *
+ * A rule returning a value when the assertion passes and another value when
+ * the assertion does not pass.
+ *
+ * @copyright  Copyright © 2007-2016 Hoa community
+ * @license    New BSD License
+ */
+class ThenElse implements Rule
+{
+    /**
+     * @var Then
+     */
+    private $rule;
+
+    /**
+     * @var mixed
+     */
+    private $otherwise;
+
+    /**
+     * @param string $rule
+     * @param mixed  $result
+     * @param mixed  $otherwise
+     */
+    public function __construct($rule, $result, $otherwise)
+    {
+        $this->rule      = new Then($rule, $result);
+        $this->otherwise = $otherwise;
+    }
+
+    /**
+     * @param Ruler   $ruler
+     * @param Context $context
+     *
+     * @return bool
+     */
+    public function valid(Ruler $ruler, Context $context)
+    {
+        return $this->rule->valid($ruler, $context);
+    }
+
+    /**
+     * @param Ruler   $ruler
+     * @param Context $context
+     *
+     * @return mixed
+     */
+    public function execute(Ruler $ruler, Context $context)
+    {
+        try {
+            return $this->rule->execute($ruler, $context);
+        } catch (RuleDoesNotValidate $e) {
+            return $this->otherwise;
+        }
+    }
+}
diff --git a/composer.json b/composer.json
index 2a393fc..ec155ec 100644
--- a/composer.json
+++ b/composer.json
@@ -28,6 +28,7 @@
         "hoa/consistency": "~1.0",
         "hoa/exception"  : "~1.0",
         "hoa/file"       : "~1.0",
+        "hoa/heap"       : "~0.0@dev",
         "hoa/protocol"   : "~1.0",
         "hoa/visitor"    : "~2.0"
     },