Extrapolate spline

returns a spline of order `g`

= fnxtr(`f`

,`order`

)`order`

that extrapolates the spline
`f`

. `g`

equals `f`

on its basic
interval, but `g`

is a polynomial of the given order outside that
interval. `g`

satisfies at least `order`

smoothness
conditions at the ends of the basic interval of `f`

, that is, at the new
breaks. It is most useful to use a positive value of `order`

that is
smaller than the order of `f`

.

uses an
extrapolation order equal to 2. It is equivalent to `g`

= fnxtr(`f`

)`fnxtr(f,2)`

.