Some features for more flexible LESS

take an example
```
.border-top-radius(@radius) {
   border-top-right-radius: max(0, unit(@radius));
   border-top-left-radius: max(0, unit(@radius));
 }
```
since i dint know the unit i had to remove it with unit and then compare. So, i need to now put the unit. i can use unit only function.
```
.border-top-radius(@radius) {
   @unit: unitonly(@radius);
   border-top-right-radius: max(unit(0,@unit), @radius);
   border-top-left-radius: max(unit(0,@unit), @radius);
 }
```

now some times we write only 0 without units. In that case we can use. zero(). I don't know what max will return. But by using zero function zero with any unit will be converted to only 0. rest values remain unaltered.
```
border-top-right-radius: zero(max(0px, @radius));
border-top-left-radius: zero(max(0px, @radius));
```