 Geometric-attempt exponential backoff

A packet with $k$ previous collisions attempts independently in each slot with probability $p_k=p_0{2}^{-k}$. On failure it advances to stage $k+1$, and on success it leaves. The Markov chain state records the number of packets at every stage, rather than only the total backlog. This memoryless-window model is related to binary exponential backoff, but its geometric waiting times differ from uniform counters in finite retry windows.

// Target: probability-and-statistics.bigb