Drupal 8.0.0 beta 12. More info: https://www.drupal.org/node/2514176

core/vendor/sebastian/recursion-context/README.md

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+# Recursion Context
+
+...
+
+## Installation
+
+To add Recursion Context as a local, per-project dependency to your project, simply add a dependency on `sebastian/recursion-context` to your project's `composer.json` file. Here is a minimal example of a `composer.json` file that just defines a dependency on Recursion Context 1.0:
+
+    {
+        "require": {
+            "sebastian/recursion-context": "~1.0"
+        }
+    }