A linear mixed-effects structure is a list of model components
representing different sets of parameters in the linear mixed-effects
model. An lmeStruct list must contain at least a
reStruct object, but may also contain corStruct and
varFunc objects. NULL arguments are not included in the
lmeStruct(reStruct, corStruct, varStruct)
a reStruct representing a random effects
an optional corStruct object, representing a
correlation structure. Default is NULL.
an optional varFunc object, representing a
variance function structure. Default is NULL.
a list of model components determining the parameters to be estimated
for the associated linear mixed-effects model.