I have written this in serious code before, and I don't regret it:

    from itertools import chain

    flatten = chain.from_iterable
    def lflatten(x): return list(flatten(x))
    def sflatten(x): return set(flatten(x))

I know it exists, but the universally known name is 80% of the utility of having a standard flatten() function.

I could tolerate a slightly different name, but `itertools.chain.from_iterable` is absurd to me.