This is a special function used in the context of the Cox survival model.
It identifies stratification variables when they appear on the right hand
side of a formula.
USAGE:
strata(..., na.group=F)
REQUIRED ARGUMENTS:
...:
any number of variables. All must be the same length.
OPTIONAL ARGUMENTS:
na.group:
a logical variable, if TRUE, then missing values are treated as a
distinct level of each variable.
VALUE:
a new factor, whose levels are all possible combinations of the factors
supplied as arguments.
DETAILS:
The result is identical to the interaction function,
but for the labeling of the factors (strata is more verbose).