An implementation of this method is used to select messages off an actor's queue for the purpose of selective receive. If the message
is selected (i.e. it should be processed now), then this method should return a non-null value. If the message
is to be skipped, this method should return
. The value returned by this method, will be returned by the
this instance has been passed to.
If the message is selected, this method may process it (and may even call
for nested selective receives),
or it may choose to return the message and have it processed when it is returned from the enclosing