function Zksτ(k, two_s, two_τ, two_λs, two_λs′, σs, tbs; refζs=(1,2,3,1))
    i,j = ij_from_k(k)
    #
    ms² = tbs.ms^2
    two_js = tbs.two_js
    two_j0 = two_js[4]
    two_λ0 = two_λs[4]
    # 
    w0 = wr(k,refζs[4],0);
    wi = wr(k,refζs[i],i);
    wj = wr(k,refζs[j],j);
    wk = wr(k,refζs[k],k);
    # 
    cosθ = cosθij(k,σs,ms²)
    #
    val =   phase(tbs.two_js[k]-two_λs′[k]) * # particle-2 convention
        sqrt(two_s+1) * wignerd_doublearg(two_s, two_τ, two_λs′[i]-two_λs′[j], cosθ) *
            phase(tbs.two_js[j]-two_λs′[j]) # particle-2 convention
    # 
    val *= two_j0==0 ? (two_τ == two_λs′[k]) : wignerd_doublearg_sign(two_j0, two_λ0, two_τ-two_λs′[k], cosζ(w0, σs, ms²), ispositive(w0))
    val *= two_js[i]==0 ? 1 : wignerd_doublearg_sign(two_js[i], two_λs′[i], two_λs[i], cosζ(wi, σs, ms²), ispositive(wi))
    val *= two_js[j]==0 ? 1 : wignerd_doublearg_sign(two_js[j], two_λs′[j], two_λs[j], cosζ(wj, σs, ms²), ispositive(wj))
    val *= two_js[k]==0 ? 1 : wignerd_doublearg_sign(two_js[k], two_λs′[k], two_λs[k], cosζ(wk, σs, ms²), ispositive(wk))
    # 
    return val
end

wignerd_doublearg_sign(two_j,two_λ1, two_λ2, cosθ, ispositive) =
    (ispositive ? 1 : x"-1"^div(two_λ1-two_λ2, 2)) *
        wignerd_doublearg(two_j, two_λ1, two_λ2, cosθ)
