bug on opa mongodb ?

counter.opa

database test {
  int /counter = 0;
  stringmap(int) /mymap;
}
function action(_) {
    Int.repeat(function (_) {
	c = /test/counter
	/test/mymap["{c}"] <- c
	/test/counter = c + 1
	void
	}, void, 200);
    #msg = <>counter : {/test/counter}!</>;
}
function show_map(_) {
	c = Map.fold(function(key,value,acc) {
		acc+1;
	}, /test/mymap, 0);
	#msg_map = <>rows : {c}</>;
}
function page() {
    <h1>Hello, world</h1>
    <div id="msg">counter : {/test/counter} !</div>
    <button onclick={action}>inc 200</button>
    <div id="msg_map"></div>
    <button onclick={show_map}>show map</button>;
}
Server.start(
    Server.http,
    { ~page, title: "Hello, world" }
);