Index: lib/common.js
===================================================================
--- a/lib/common.js
+++ b/lib/common.js
@@ -96,8 +96,27 @@
     }
   }
 
   selectors.push(selector.substring(start));
   return selectors;
 }
 
 exports.splitSelector = splitSelector;
+
+/**
+ * Yields subdomains of a domain
+ * @param {string} domain
+ * @yields {string}
+ */
+function* subdomains(domain)
+{
+  while (true)
+  {
+    let index = domain.indexOf(".");
+    if (index == -1)
+      break;
+
+    yield domain = domain.substring(index + 1);
+  }
+}
+
+exports.subdomains = subdomains;