Given

```
userplays = { "Alice"   : { "AC/DC" : 2,
                            "The Raconteurs" : 3,
                            "Mogwai" : 1
                          },
              "Bob"     : { "The XX" : 4,
                            "Lady Gaga" : 3,
                            "Mogwai" : 1,
                            "The Raconteurs" : 1
                          },
              "Charlie" : { "AC/DC" : 7,
                            "Lady Gaga" : 7
                          }
            }

```

get a list of all bands:

```
['Lady Gaga', 'Mogwai', 'AC/DC', 'The Raconteurs', 'The XX']

```

I can do 

```
list(set(flatten([ [ band 
                     for band 
                     in playcounts.keys() ] 
                   for playcounts 
                   in userplays.values() ] ) ) )

```

where `flatten` is from [Flatten (an irregular) list of lists](https://stackoverflow.com/questions/2158395/flatten-an-irregular-list-of-lists-in-python), but is it possible without `flatten`, using only list/dict comprehensions?