diff --git a/src/Phpml/Classification/Classifier.php b/src/Phpml/Classification/Classifier.php
index 00e6779..1477904 100644
--- a/src/Phpml/Classification/Classifier.php
+++ b/src/Phpml/Classification/Classifier.php
@@ -4,18 +4,8 @@ declare (strict_types = 1);
 
 namespace Phpml\Classification;
 
-interface Classifier
-{
-    /**
-     * @param array $samples
-     * @param array $labels
-     */
-    public function train(array $samples, array $labels);
+use Phpml\Estimator;
 
-    /**
-     * @param array $samples
-     *
-     * @return mixed
-     */
-    public function predict(array $samples);
+interface Classifier extends Estimator
+{
 }
diff --git a/src/Phpml/Estimator.php b/src/Phpml/Estimator.php
new file mode 100644
index 0000000..0631679
--- /dev/null
+++ b/src/Phpml/Estimator.php
@@ -0,0 +1,21 @@
+<?php
+
+declare (strict_types = 1);
+
+namespace Phpml;
+
+interface Estimator
+{
+    /**
+     * @param array $samples
+     * @param array $targets
+     */
+    public function train(array $samples, array $targets);
+
+    /**
+     * @param array $samples
+     *
+     * @return mixed
+     */
+    public function predict(array $samples);
+}
diff --git a/src/Phpml/FeatureExtraction/TfIdfTransformer.php b/src/Phpml/FeatureExtraction/TfIdfTransformer.php
index 152919e..fade5b4 100644
--- a/src/Phpml/FeatureExtraction/TfIdfTransformer.php
+++ b/src/Phpml/FeatureExtraction/TfIdfTransformer.php
@@ -4,6 +4,8 @@ declare (strict_types = 1);
 
 namespace Phpml\FeatureExtraction;
 
+use Phpml\Transformer;
+
 class TfIdfTransformer implements Transformer
 {
     /**
diff --git a/src/Phpml/FeatureExtraction/TokenCountVectorizer.php b/src/Phpml/FeatureExtraction/TokenCountVectorizer.php
index 95ecc33..5273f6b 100644
--- a/src/Phpml/FeatureExtraction/TokenCountVectorizer.php
+++ b/src/Phpml/FeatureExtraction/TokenCountVectorizer.php
@@ -5,6 +5,7 @@ declare (strict_types = 1);
 namespace Phpml\FeatureExtraction;
 
 use Phpml\Tokenization\Tokenizer;
+use Phpml\Transformer;
 
 class TokenCountVectorizer implements Transformer
 {
diff --git a/src/Phpml/Pipeline.php b/src/Phpml/Pipeline.php
new file mode 100644
index 0000000..2ac0ed0
--- /dev/null
+++ b/src/Phpml/Pipeline.php
@@ -0,0 +1,29 @@
+<?php
+
+declare (strict_types = 1);
+
+namespace Phpml\Pipeline;
+
+class Pipeline
+{
+    /**
+     * @var array
+     */
+    private $stages;
+
+    /**
+     * @param array $stages
+     */
+    public function __construct(array $stages)
+    {
+        $this->stages = $stages;
+    }
+
+    /**
+     * @param mixed $stage
+     */
+    public function addStage($stage)
+    {
+        $this->stages[] = $stage;
+    }
+}
diff --git a/src/Phpml/Regression/Regression.php b/src/Phpml/Regression/Regression.php
index 12d2f52..27a1b83 100644
--- a/src/Phpml/Regression/Regression.php
+++ b/src/Phpml/Regression/Regression.php
@@ -4,18 +4,8 @@ declare (strict_types = 1);
 
 namespace Phpml\Regression;
 
-interface Regression
-{
-    /**
-     * @param array $samples
-     * @param array $targets
-     */
-    public function train(array $samples, array $targets);
+use Phpml\Estimator;
 
-    /**
-     * @param array $samples
-     *
-     * @return mixed
-     */
-    public function predict(array $samples);
+interface Regression extends Estimator
+{
 }
diff --git a/src/Phpml/FeatureExtraction/Transformer.php b/src/Phpml/Transformer.php
similarity index 84%
rename from src/Phpml/FeatureExtraction/Transformer.php
rename to src/Phpml/Transformer.php
index 13eb955..bdc809b 100644
--- a/src/Phpml/FeatureExtraction/Transformer.php
+++ b/src/Phpml/Transformer.php
@@ -2,7 +2,7 @@
 
 declare (strict_types = 1);
 
-namespace Phpml\FeatureExtraction;
+namespace Phpml;
 
 interface Transformer
 {