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diff --git a/js/mathjax/extensions/TeX/extpfeil.js b/js/mathjax/extensions/TeX/extpfeil.js
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+++ b/js/mathjax/extensions/TeX/extpfeil.js
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+// @license magnet:?xt=urn:btih:8e4f440f4c65981c5bf93c76d35135ba5064d8b7dn=apache-2.0.txt Apache-2.0
+/* -*- Mode: Javascript; indent-tabs-mode:nil; js-indent-level: 2 -*- */
+/* vim: set ts=2 et sw=2 tw=80: */
+
+/*************************************************************
+ *
+ *  MathJax/extensions/TeX/extpfeil.js
+ *  
+ *  Implements additional stretchy arrow macros.
+ *
+ *  ---------------------------------------------------------------------
+ *  
+ *  Copyright (c) 2011-2020 The MathJax Consortium
+ * 
+ *  Licensed under the Apache License, Version 2.0 (the "License");
+ *  you may not use this file except in compliance with the License.
+ *  You may obtain a copy of the License at
+ * 
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ * 
+ *  Unless required by applicable law or agreed to in writing, software
+ *  distributed under the License is distributed on an "AS IS" BASIS,
+ *  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ *  See the License for the specific language governing permissions and
+ *  limitations under the License.
+ */
+
+MathJax.Extension["TeX/extpfeil"] = {
+  version: "2.7.9"
+};
+
+MathJax.Hub.Register.StartupHook("TeX Jax Ready",function () {
+  
+  var TEX = MathJax.InputJax.TeX,
+      TEXDEF = TEX.Definitions;
+  
+  //
+  //  Define the arrows to load the AMSmath extension
+  //  (since they need its xArrow method)
+  // 
+  TEXDEF.Add({
+    macros: {
+      xtwoheadrightarrow: ['Extension','AMSmath'],
+      xtwoheadleftarrow:  ['Extension','AMSmath'],
+      xmapsto:            ['Extension','AMSmath'],
+      xlongequal:         ['Extension','AMSmath'],
+      xtofrom:            ['Extension','AMSmath'],
+      Newextarrow:        ['Extension','AMSmath']
+    }
+  },null,true);
+  
+  //
+  //  Redefine the macros when AMSmath is loaded
+  //
+  MathJax.Hub.Register.StartupHook("TeX AMSmath Ready",function () {
+    MathJax.Hub.Insert(TEXDEF,{
+      macros: {
+        xtwoheadrightarrow: ['xArrow',0x21A0,12,16],
+        xtwoheadleftarrow:  ['xArrow',0x219E,17,13],
+        xmapsto:            ['xArrow',0x21A6,6,7],
+        xlongequal:         ['xArrow',0x003D,7,7],
+        xtofrom:            ['xArrow',0x21C4,12,12],
+        Newextarrow:        'NewExtArrow'
+      }
+    });
+  });
+
+  //
+  //  Implements \Newextarrow to define a new arrow (not compatible with \newextarrow, but
+  //  the equivalent for MathJax)
+  //
+  TEX.Parse.Augment({
+    NewExtArrow: function (name) {
+      var cs    = this.GetArgument(name),
+          space = this.GetArgument(name),
+          chr   = this.GetArgument(name);
+      if (!cs.match(/^\\([a-z]+|.)$/i)) {
+        TEX.Error(["NewextarrowArg1",
+                   "First argument to %1 must be a control sequence name",name]);
+      }
+      if (!space.match(/^(\d+),(\d+)$/)) {
+        TEX.Error(
+          ["NewextarrowArg2",
+           "Second argument to %1 must be two integers separated by a comma",
+           name]
+        );
+      }
+      if (!chr.match(/^(\d+|0x[0-9A-F]+)$/i)) {
+        TEX.Error(
+          ["NewextarrowArg3",
+           "Third argument to %1 must be a unicode character number",
+           name]
+        );
+      }
+      cs = cs.substr(1); space = space.split(","); chr = parseInt(chr);
+      this.setDef(cs, ['xArrow', chr, parseInt(space[0]), parseInt(space[1])]);
+    }
+  });
+  
+  MathJax.Hub.Startup.signal.Post("TeX extpfeil Ready");
+});
+
+MathJax.Ajax.loadComplete("[MathJax]/extensions/TeX/extpfeil.js");
+// @license-end