Reference Error on using prune

I added the package 'underscorestring:underscore.string" to my meteor project and on trying to use the "prune" method as below i am getting Reference Error. I could not find any document on how to invoke any methods. Can you please point out what i am doing wrong here?

Template.registerHelper('prune', function(string, length, pruneString)
{
console.log('string = ' + string);
console.log('length = ' + length);
console.log('pruneString = ' + pruneString);

```
return prune(string, length, pruneString);
```

});

Thanks

Jay