Function to partition a set according to a predicate.

partition.jl

"""
    partition(f, xs)

Partitions a collection according to a predicate `f`. Similar to `filter`, but returns
the both the elements for which `f` is `true` and for which it is `false`, in two different
collections.

Returns a tuple of vectors, where the first vector contains the elements for which `f` is true
and the second the elements for which `f` is false.
"""
function partition(f, xs)
    T = eltype(xs)
    xs_true, xs_false = T[], T[]
    for x in xs
        f(x) ? push!(xs_true, x) : push!(xs_false, x)
    end
    return xs_true, xs_false
end