Figure 3.

Urn model with triggering. (A) An element that had previously been drawn from the urn, is drawn again: the element is added to $\mathcal{S}$ and it is put back in the urn along with $\rho$ additional copies of it. (B) An element that never appeared in the sequence is drawn: the element is added to $\mathcal{S}$, put back in the urn along with $\rho$ additional copies of it, and $\nu +1$ brand new and distinct balls are also added to the urn.