Here's what you're looking for:

```
//http://css-tricks.com/remux-an-experimental-approach-to-responsive-web-design/
@rem-fallback: true; // switch to include px fallback

.rem (@px-value) {
	.rem(@px-value, (unit(@px-value) + 1));
}

.rem (@px-value-sm, @px-value-lg) {
    @rem-value-sm: ((round((@px-value-sm / unit(@base-font-size-sm)) * 10000000)) / 10000000);
    @rem-value-lg: ((round((@px-value-lg / unit(@base-font-size-lg)) * 10000000)) / 10000000);
    
    .result (@rem-value, @px-value, @rem-fallback) when (@rem-fallback = true) {
        font-size: ~"@{px-value}px";
        font-size: ~"@{rem-value}rem";
    }
    .result (@rem-value, @px-value, @rem-fallback) when (@rem-fallback = false) {
        font-size: ~"@{rem-value}rem";
    }
    
    .result(@rem-value-sm, unit(@px-value-sm), @rem-fallback);
    @media @bp2 {
    	.result(@rem-value-lg, unit(@px-value-lg), @rem-fallback);
    }
}
```