Javascript to delete last 4 bookmarks of every 10 bookmarks

if (app.viewerVersion < 10) {
app.addMenuItem({ cName: "RemBM last4every10", cUser: "RemBM last4every10", cParent: "Tools", 
cExec: "Removelast4ofevery10BM(this)", cEnable: "event.rc = (event.target != null);"}); 
} else {
app.addToolButton({ cName: "RemBM last4every10", cLabel: "RemBM last4every10", cTooltext: "RemBM last4every10", 
cExec: "Removelast4ofevery10BM(this)", cEnable: "event.rc = (event.target != null);"}); 
}
/* Remove last 4 of every 10 BM in the document */
function Removelast4ofevery10BM(doc) {
   console.clear(); console.show();
   var bm = this.bookmarkRoot;
   var bmlength = bm.children[0].children[0].children.length;
   try {
      for (var i = 0; i < bmlength; i++) {
          if (i % 10 >= 6) { // Remove the last 4 bookmarks out of every 10
              bm.children[0].children[0].children[i].remove();
          }
      }
   }
   catch(e) {
   app.alert("Processing error: "+e)
   }
}