@uml
Modify the current {@link IntSet} object to contain all elements that are
present in itself, the specified {@code set}, or both.
@param set The set to add to the current set. A {@code null} argument is
treated as though it were an empty set.
@return {@code this} (to support chained calls)
@exception IllegalStateException if the current set is read-only
@uml Modify the current {@link IntSet} object to contain all elements that are present in itself, the specified {@code set}, or both.
@param set The set to add to the current set. A {@code null} argument is treated as though it were an empty set. @return {@code this} (to support chained calls)
@exception IllegalStateException if the current set is read-only