This should be added here
```sass
@right: right;
@left:  left;
@dir:   ltr;
```
and this in `rtl,less` file
```sass
@right: right;
@left:  left;
@dir:   ltr;
```

Then compile
`bootstrap.less` => `bootstrap.css`
`rtl.less` => `bootstrap.rtl.css`