an object specifying the covariance structure.

a matrix whose columns represent locations.

for noisy observations : an optional vector containing the noise variance at each observation

a matrix representing the covariance matrix for the locations specified in the X argument, including a possible nugget effect or observation noise.

a vector of length n (X size) containing a replication of the nugget effet or the observation noise (so that C-diag(vn) contains the covariance matrix when there is no nugget effect nor observation noise)