Fill in union context

lib/Context/Context.php

@@ -6,8 +6,7 @@
 declare(strict_types=1);
 namespace JKingWeb\Arsse\Context;
 
-class Context extends AbstractContext {
-    use RootMembers;
+class Context extends RootContext {
     use BooleanMembers;
     use ExclusionMembers;
 

lib/Context/RootContext.php

@@ -6,7 +6,7 @@
 declare(strict_types=1);
 namespace JKingWeb\Arsse\Context;
 
-trait RootMembers {
+class RootContext extends AbstractContext {
     public $limit = 0;
     public $offset = 0;
 

lib/Context/UnionContext.php

@@ -0,0 +1,41 @@
+<?php
+/** @license MIT
+ * Copyright 2017 J. King, Dustin Wilson et al.
+ * See LICENSE and AUTHORS files for details */
+
+declare(strict_types=1);
+namespace JKingWeb\Arsse\Context;
+
+class UnionContext extends RootContext implements \ArrayAccess, \Countable, \IteratorAggregate {
+    protected $contexts = [];
+
+    public function offsetExists(mixed $offset): bool {
+        return array_key_exists($offset, $this->contexts);
+    }
+
+    public function offsetGet(mixed $offset): mixed {
+        return $this->contexts[$offset] ?? null;
+    }
+
+    public function offsetSet(mixed $offset, mixed $value): void {
+        $this->contexts[$offset ?? count($this->contexts)] = $value;
+    }
+
+    public function offsetUnset(mixed $offset): void {
+        unset($this->contexts[$offset]);
+    }
+
+    public function count(): int {
+        return count($this->contexts);
+    }
+
+    public function getIterator(): \Traversable {
+        foreach ($this->contexts as $k => $c) {
+            yield $k => $c;
+        }
+    }
+
+    public function __construct(Context ...$context) {
+        $this->contexts = $context;
+    }
+}