A one sided formula of the form
~ t, or
~ t | g,
specifying a time covariate
t and, optionally, a grouping factor
A covariate for this correlation structure must be integer valued.
When a grouping factor is present in
formula, the correlation structure
is assumed to apply only to observations within the same grouping level;
observations with different grouping levels are assumed to be uncorrelated.
~ 1, which corresponds to using the order of the observations
in the data as a covariate, and no groups.