form
a one sided formula of the form ~ t, or ~ t |
g, specifying a time covariate t and, optionally, a
grouping factor g. When a grouping factor is present in
form, the correlation st
fixed
an optional logical value indicating whether the
coefficients should be allowed to vary in the optimization, or kept
fixed at their initial value. Defaults to FALSE, in which case
the coefficients are allowed to vary.