We investigate pattern avoidance in permutations satisfying some additional
restrictions. These are naturally considered in terms of avoiding patterns in
linear extensions of certain forest-like partially ordered sets, which we call
binary shrub forests. In this context, we enumerate forests avoiding patterns
of length three. In four of the five non-equivalent cases, we present explicit
enumerations by exhibiting bijections with certain lattice paths bounded above
by the line $y=\ell x$, for some $\ell\in\mathbb{Q}^+$, one of these being the
celebrated Duchon's club paths with $\ell=2/3$. In the remaining case, we use
the machinery of analytic combinatorics to determine the minimal polynomial of
its generating function, and deduce its growth rate.
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