gu function
gu(u, sigma, phi, model, additionalparameters)
distance
variance parameter, see Brix and Diggle (2001)
scale parameter, see Brix and Diggle (2001)
correlation type, see ?CovarianceFct
vector of additional parameters for certain classes of covariance function (eg Matern), these must be supplied in the order given in ?CovarianceFct
this is just a wrapper for CovarianceFct