buildHTML's buildExpression is destructive, changing the tree object, this clones the object first before manipulating it.

src/buildHTML.js

@@ -1157,6 +1157,10 @@ var buildGroup = function(group, options, prev) {
  * nodes.
  */
 var buildHTML = function(tree, settings) {
+    // buildExpression is destructive, so we need to make a clone
+    // of the incoming tree so that it isn't accidentally changed
+    tree = JSON.parse(JSON.stringify(tree));
+
     var startStyle = Style.TEXT;
     if (settings.displayMode) {
         startStyle = Style.DISPLAY;