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We consider ${\cal S}$, the class of finite semilattices; ${\cal T}$, the class of finite treeable semilattices; and ${{\cal T}_m}$, the subclass of ${\cal T}$ which contains trees with branching bounded by m. We prove that ${\cal E}{\cal S}$, the class of finite lattices with linear extensions, is a Ramsey class. We calculate Ramsey degrees for structures in ${\cal S}$, ${\cal T}$, and ${{\cal T}_m}$. In addition to this we give a topological interpretation of our results and we apply our result to canonization of linear orderings on finite semilattices. In particular, we give an example of a Fraïssé class ${\cal K}$ which is not a Hrushovski class, and for which the automorphism group of the Fraïssé limit of ${\cal K}$ is not extremely amenable (with the infinite universal minimal flow) but is uniquely ergodic.
In this paper we prove two main results. The first is a necessary and sufficient condition for a semidirect product of a semilattice by a group to be finitely generated. The second result is a necessary and sufficient condition for such a semidirect product to be finitely presented.
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