jsyaml.addConstructor fails (at least in browserified versions)

I’m trying to add a constructor for custom tags using

``` js
jsyaml.addConstructor(
  '!sometag',
  function(node) { return "test"} );
```

This fails with "(Loader || _loader.Loader).addConstructor is not a function". I checked _loader.Loader using Firebug, and it indeed does not have that function. If it’s not a bug, do you have any hints on what I am doing wrong?
