TrimmedMean.
psi
The psi function for least trimmed mean
The analytic derivative of rho
z : array_like
1d array
psi : ndarray
psi(z) = z for |z| <= c psi(z) = 0 for |z| > c
psi(z) = z for |z| <= c
psi(z) = 0 for |z| > c