The psi function for least trimmed mean
The analytic derivative of rho
z : array-like
1d array
array :
psi(z) = z for |z| <= c psi(z) = 0 for |z| > c
scikits.statsmodels.robust.norms.TrimmedMean
scikits.statsmodels.robust.norms.TrimmedMean.psi_deriv
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