(where someSequence is a sequence of sequences) results in two equal sequences (apart from their exact types). I would like to have flatten() renamed to joined(), resulting in joined(_ seemingly having an empty array as the default parameter.
Similarly, I propose to give joined(_ for a sequence of strings the empty string as the default parameter, allowing us to replace something like
["Hello, ", "world!"].joined(separator: "") // "Hello, world!"
["Hello, ", "world!"].joined() // "Hello, world!”
In other words, I think joined(_ should have a default parameter of either `` or `””` and flatten() could then be removed as it would act the exact same way as joined().