psi.weight(svals, ips = 1, xk = 1.06)
rho.weight(svals, ips = 1, xk = 1.06)
psp.weight(svals, ips = 1, xk = 1.06)
chi.weight(svals, ips = 1, xk = 1.06)
ips = 1
for optimal function, ips = 2
for rescaled bisquare function, and ips = 3
for Huber function.