Use the @options.smooth directly if the value's type is "boolean".

```coffee
@options.smooth is true or @options.ykeys[i] in @options.smooth
```

This code is compiled to

```javascript
var __indexOf = [].indexOf;
typeof this.options.smooth === "boolean" ? this.options.smooth : (_ref1 = this.options.ykeys[i], __indexOf.call(this.options.smooth, _ref1) >= 0);
```

In modern browsers or in v8, `[].indexOf.call(false, value)` returns false.
But, if Array.prototype.indexOf is implemented (often in IE8) by user, that code sometimes makes error.

I think that should check by `typeof @options.smooth is "boolean"`, instead of `is true`.