- which is the rank of the observation. SUBREADER must be sorted
- on V. W is the weight variable of the dictionary containing V,
- or NULL if there is no weight variable.
+ which is the rank of the observation. W is the weight variable
+ of the dictionary containing V, or NULL if there is no weight
+ variable.
+
+ The following preconditions must be met:
+
+ 1. SUBREADER must be sorted on V.
+
+ 2. The weight variables, must be non-negative.
+
+ If either of these preconditions are not satisfied, then the rank
+ variables may not be correct. In this case, if ERR is non-null,
+ it will be set according to the erroneous conditions encountered.