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# Implementation bodies for unique and unique!
function _unique(itr, f::Union{Nothing, Callable} = nothing)
    T = @default_eltype(itr)
    y = iterate(itr)
    y === nothing && return T[]
    x, st = y
    R = !isconcretetype(T) && IteratorEltype(itr) == EltypeUnknown() ? typeof(x) : T
    f === nothing && return __uniquify(itr, R[], Set{R}(), st, x)
    fx = f(x)
    return __uniquify(itr, R[], Set{typeof(fx)}(), st, x, f, fx)