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diff --git a/bg/ExternalLicenses.js b/bg/ExternalLicenses.js
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--- /dev/null
+++ b/bg/ExternalLicenses.js
@@ -0,0 +1,96 @@
+/**
+* GNU LibreJS - A browser add-on to block nonfree nontrivial JavaScript.
+*
+* Copyright (C) 2018 Giorgio Maone <giorgio@maone.net>
+*
+* This file is part of GNU LibreJS.
+*
+* GNU LibreJS is free software: you can redistribute it and/or modify
+* it under the terms of the GNU General Public License as published by
+* the Free Software Foundation, either version 3 of the License, or
+* (at your option) any later version.
+*
+* GNU LibreJS is distributed in the hope that it will be useful,
+* but WITHOUT ANY WARRANTY; without even the implied warranty of
+* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+* GNU General Public License for more details.
+*
+* You should have received a copy of the GNU General Public License
+* along with GNU LibreJS.  If not, see <http://www.gnu.org/licenses/>.
+*/
+
+/**
+  Singleton to handle external licenses, e.g. WebLabels
+*/
+
+"use strict";
+
+let licensesByURL = new Map();
+{
+  let {licenses} = require("../license_definitions");
+  for (let l of Object.values(licenses).filter(l => l.canonicalUrl)) {
+    for (let url of l.canonicalUrl) {
+      licensesByURL.set(url, l);
+    }
+  }
+}
+
+var ExternalLicenses = {
+  async check(script) {
+    let {url, tabId, frameId} = script;
+    let scriptInfo = await browser.tabs.sendMessage(tabId, {
+      action: "checkLicensedScript",
+      url
+    }, {frameId});
+    if (!(scriptInfo && scriptInfo.licenseURLs.length)) {
+      return null;
+    }
+    scriptInfo.licenses = new Set();
+    scriptInfo.allFree = true;
+    scriptInfo.toString = function() {
+      let licenseIds = [...this.licenses].map(l => l.identifier).sort().join(", ");
+      return this.allFree ? `Free license${licenseIds.length > 1 ? "s" : ""} (${licenseIds})` : `Mixed free (${licenseIds}) and unknown licenses`;
+    }
+    
+    for (let u of scriptInfo.licenseURLs) {
+      if (licensesByURL.has(u)) {
+        scriptInfo.licenses.add(licensesByURL.get(u));
+      } else {
+        scriptInfo.allFree = false;
+        break;
+      }
+    }
+    return scriptInfo;
+  },
+  
+  /**
+  * moves / creates external license references before any script in the page
+  * if needed, to have them ready when the first script load is triggered
+  * Returns true if the document has been actually modified, false otherwise.
+  */
+  optimizeDocument(document) {
+    let link = document.querySelector(`link[rel="jslicense"], link[data-jslicense="1"], a[rel="jslicense"], a[data-jslicense="1"]`);
+    if (link) {
+      let move = () => !!document.head.insertBefore(link, document.head.firstChild);
+      if (link.parentNode === document.head) {
+        for (let node; node = link.previousElementSibling;) {
+          if (node.tagName.toUpperCase() === "SCRIPT") {
+            return move();
+          }
+        }
+      } else { // the reference is only in the body
+        if (link.tagName.toUpperCase() === "A") {
+          let newLink = document.createElement("link");
+          newLink.rel = "jslicense";
+          newLink.setAttribute("href", link.getAttribute("href"));
+          link = newLink;
+        }
+        return move();
+      }
+    }
+    return false;
+  }
+};
+
+
+module.exports = { ExternalLicenses };