public boolean add( final int i ) {
    RangeUtil.checkPositiveOrZero(i, "i");
    return _nbsi.add(i);
  }
Add {@code i} to the set.  This is the lower-case '{@code int}' version
of {@link #add} - no autoboxing.  Negative values throw
IllegalArgumentException.
@throws IllegalArgumentException if i is negative.
@return <tt>true</tt> if i was added to the set.
jctools-core/src/main/java/org/jctools/maps/NonBlockingSetInt.java