flatten & select

[guzhenhuaGitHub]

I tried this in playground:

let count = [1, 2, 3, nil, nil, 4, 5, 6, 7]
.flatten()

and I got an error:

Playground execution failed:

error: RbSwift.playground:1:6: error: cannot invoke 'flatten' with no arguments
.flatten()
^

and I tried this in playground:

let count = [1, 2, 3, nil, nil, 4, 5, 6, 7]
.flatten()
.select { true }

and I got count was a :

[]