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.