Merge pull request #796 from mathjax/fix-chtml-css

Prevent CHTML adaptice CSS from adding character CSS multiple times

ts/output/chtml/Usage.ts

@@ -23,14 +23,13 @@
 
 /**
  * Class used for tracking usage of font characters or wrappers
- *  (should extend Set<T>, but that doesn't work for compiling to ES2015).
  */
 export class Usage<T> {
 
   /**
    * The used items.
    */
-  protected used: Set<T> = new Set<T>();
+  protected used: Set<string> = new Set<string>();
 
   /**
    * The items marked as used since last update.
@@ -41,18 +40,19 @@ export class Usage<T> {
    * @param {T} item   The item that has been used
    */
   public add(item: T) {
-    if (!this.used.has(item)) {
+    const name = JSON.stringify(item);
+    if (!this.used.has(name)) {
       this.needsUpdate.push(item);
     }
-    this.used.add(item);
+    this.used.add(name);
   }
 
   /**
    * @param {T} item     The item to check for being used
    * @return {boolean}   True if the item has been used
    */
   public has(item: T): boolean {
-    return this.used.has(item);
+    return this.used.has(JSON.stringify(item));
   }
 
   /**