I always thought that "hidden" meant, that the listener does not know about the hidden signals at all.
But I do not know if this is true. Does "hidden" imply that the listener does not know about the hidden
signals at all? Or does it mean the listener might know that there are hidden reference and anchors but
he/she does not know where they are? And if he/she might know, should he know,
i.e. should it be told a priori?
