Expand out the data frame for a nested factor such as (1| a/b).
This is used by the variance function routines of
A data frame containing the nesting variables
The initialize function of a
coxme variance family is
passed, as one of its arguments, a data frame
the grouping variables, each of which is a factor..
Assume a nested factor
(1| a/b) in the model formula and
a data set whose first few rows are:
1 2 3 4 5
a b 1 1 1 2 2 1
The function will replace the second column with a variable named
a/b and values of 1/1, 1/2, 2/1, etc.
an updated data frame