parameter for the decreasing step function after which the step decreases.
Author
Christophe Dutang
Details
The decrstep function is a decreasing step serie such that decrstep(k)
equals to \(1/2/(k - param)\) when \(k>param\), 1/2, otherwise.
Functions decrstep5, decrstep10, decrstep20 are just wrappers of
decrstep.
The purestep function implements a constant step serie equaled to 1.