diff --git a/common/static/js/capa/symbolic_mathjax_preprocessor.js b/common/static/js/capa/symbolic_mathjax_preprocessor.js
index 19104553dc..766e5efc03 100644
--- a/common/static/js/capa/symbolic_mathjax_preprocessor.js
+++ b/common/static/js/capa/symbolic_mathjax_preprocessor.js
@@ -1,22 +1,35 @@
+/* This file defines a processor in between the student's math input
+   (AsciiMath) and what is read by MathJax. It allows for our own
+   customizations, such as use of the syntax "a_b__x" in superscripts, or
+   possibly coloring certain variables, etc&.
+
+   It is used in the <textline> definition like the following:
+
+     <symbolicresponse expect="a_b^c + b_x__d" size="30">
+       <textline math="1"
+         preprocessorClassName="SymbolicMathjaxPreprocessor"
+	 preprocessorSrc="/static/js/capa/symbolic_mathjax_preprocessor.js"/>
+     </symbolicresponse>
+*/
 window.SymbolicMathjaxPreprocessor = function () {
-    this.fn = function (eqn) {
-      // flags and config
-      var superscriptsOn = true;
+  this.fn = function (eqn) {
+    // flags and config
+    var superscriptsOn = true;
 
-      if (superscriptsOn) {
-	// find instances of "__" and make them superscripts ("^") and tag them
-	// as such. Specifcally replace instances of "__X" or "__{XYZ}" with
-	// "^{CHAR$1}", marking superscripts as different from powers
+    if (superscriptsOn) {
+      // find instances of "__" and make them superscripts ("^") and tag them
+      // as such. Specifcally replace instances of "__X" or "__{XYZ}" with
+      // "^{CHAR$1}", marking superscripts as different from powers
 
-	// a zero width space--this is an invisible character that no one would
-	// use, that gets passed through MathJax and to the server
-	var c = "\u200b";
-	eqn = eqn.replace(/__(?:([^\{])|\{([^\}]+)\})/g, '^{' + c + '$1$2}');
+      // a zero width space--this is an invisible character that no one would
+      // use, that gets passed through MathJax and to the server
+      var c = "\u200b";
+      eqn = eqn.replace(/__(?:([^\{])|\{([^\}]+)\})/g, '^{' + c + '$1$2}');
 
-	// NOTE: MathJax supports '\class{name}{mathcode}' but not for asciimath
-	// input, which is too bad. This would be preferable to the char tag
-      }
+      // NOTE: MathJax supports '\class{name}{mathcode}' but not for asciimath
+      // input, which is too bad. This would be preferable to this char tag
+    }
 
-      return eqn;
-    };
+    return eqn;
+  };
 };