This function computes an *approximation* to the \(K\) function
or the pair correlation function of a Gibbs point process.

The functions `Kmodel`

and `pcfmodel`

are generic.
The functions documented here are the methods for the class
`"ppm"`

.

The approximation is only available for stationary
pairwise-interaction models.
It uses the second order Poisson-saddlepoint approximation
(Baddeley and Nair, 2012b) which is a combination of
the Poisson-Boltzmann-Emden and Percus-Yevick approximations.

The return value is a `function`

in the R language,
which takes one argument `r`

.
Evaluation of this function, on a numeric vector `r`

,
yields values of the desired \(K\) function or pair correlation
function at these distance values.