A primary key is a variable in a dataset that has a unique value for every row. A variable must be either numeric or text type and have no duplicate or missing values. A primary key on a dataset causes appends to that dataset that have the rows with the same primary key value(s) as the first dataset to update the existing rows rather than inserting new ones.
pk(x) <- value
For the setter, a single Variable to use as the primary key or
NULL to remove the primary key.
Getter returns the Variable object that is used as the primary key
NULL if there is no primary key); setter returns
x duly modified.