A part of  Extend document is error at the "Selector Interpolation with Extend".

### Selector Interpolation with Extend
```
@variable: .bucket;
@{variable} { // interpolated selector
  color: blue;
}
.some-class:extend(.bucket) {}
```
will output 
```
.bucket,
.some-class {
  color: blue;
}
```
It's matched!!!
My version of less is 3.12.2.