Expand out the data frame for a nested factor such as (1| a/b).
This is used by the variance function routines of `coxme`

.

1 |

`x` |
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 `G`

containing
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

Terry Therneau

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