"""
undo the effect of `vcat(xs...)`
"""
function un_vcat(x::AbstractVector{T}, dim::AbstractVector{Int}) where {T}
	N = length(dim)
	xs = Array{AbstractVector{T}}(undef, N)
	for n=1:N
		i0 = sum(dim[1:(n-1)])
		xs[n] = x[(i0+1) : (i0 + dim[n])]
	end
	return xs
end

using Test: @test, @inferred
x1 = 1:4; x2 = ones(5); x3 = [7,6]
dim = [length(x1), length(x2), length(x3)]
x = vcat(x1, x2, x3)
xs = @inferred un_vcat(x, dim)
@test xs == [x1, x2, x3]