function dot2(v)
    sums = fill(0.0, size(v,1)-1)
    for i = 1:size(v,1)-1
        for j = 1:size(v,2)
            sums[i] += v[i,j]*v[i+1,j]
        end
    end
    return sums
end
function dot2Δ(v, Δ)
    g = fill(0.0, size(v)...)
    g[1,:] .+= Δ[1].*v[2,:]
    for i = 2:size(v,1)-1
        g[i,:] .+= Δ[i-1]*v[i-1,:] .+ Δ[i]*v[i+1,:]
    end
    g[end,:] .+= Δ[end].*v[end-1,:]
    return g
end

dot2(v::TrackedArray) = Flux.Tracker.track(dot2, v)
Flux.Tracker.@grad function dot2(v)
    dot2(data(v)),
    Δ-> (nobacksies(:dot2, dot2Δ(data.((v, Δ))...)) ,)
end