Returns the mean of the squared leave-one-out errors, computed with Dubrule's formula.

`leaveOneOutFun(param, model, envir = NULL)`

param

a vector containing the optimization variables.

model

an object of class `km`

.

envir

an optional environment specifying where to assign intermediate values for future gradient calculations. Default is NULL.

The mean of the squared leave-one-out errors.

F. Bachoc (2013), Cross Validation and Maximum Likelihood estimations of hyper-parameters of Gaussian processes with model misspecification. *Computational Statistics and Data Analysis*, **66**, 55-69. http://www.lpma.math.upmc.fr/pageperso/bachoc/publications.html

O. Dubrule (1983), Cross validation of Kriging in a unique neighborhood. *Mathematical Geology*, **15**, 687-699.