A one sided formula of the form `~ t`

, or `~ t | g`

,
specifying a time covariate `t`

and, optionally, a grouping factor `g`

.
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.
Defaults to `~ 1`

, which corresponds to using the order of the observations
in the data as a covariate, and no groups.