Javadoc
Adds the given number of new unarrived parties to this phaser.
If an ongoing invocation of
#onAdvance is in progress,
this method may await its completion before returning. If this
phaser has a parent, and the given number of parties is greater
than zero, and this phaser previously had no registered
parties, this child phaser is also registered with its parent.
If this phaser is terminated, the attempt to register has no
effect, and a negative value is returned.