= Incoming-edge coupling of site and bond percolation
Suppose every vertex of a <directed graph> has at most $\Delta$ incoming bonds. In <bond percolation> with parameter $q$, open a site whenever at least one incoming bond is open. Incoming bond sets have different heads and hence are disjoint, so these site indicators are <independent random variables>. Supplement their <probabilities> to $r=1-(1-q)^\Delta$. Every vertex reached by a nontrivial open <directed path> is now open. Conditioning the root to be open gives $\theta_s(r)\ge r\theta_b(q)$, since a simple <directed path> from the root never uses a bond entering the root. Thus $p_H^s\le1-(1-p_H^b)^\Delta$.
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