## unique()


A key: at most one row per combination of `cols`.


Usage

``` python
unique(*cols)
```


In `requires` it is an assumption about an input (checked on the data when the input is bound to a backend); in `ensures` it is proved by `Rel.keys_sound` from the input keys and the transformation.


## Parameters


`*cols: str`  
The names of the key's columns.


## Returns


`Unique`  
A `Unique` condition, for `requires` or `ensures`.