i = isLowerSemilattice P
The poset $P$ is a lower semilattice if every pair of vertices has a unique greatest lower bound (meet).
Clearly, the $n$ chain and the $n$ booleanLattice are lower semilattices.
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The middle ranks of the $n$ booleanLattice are not lower semilattices.
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However, the lower ranks of the $n$ booleanLattice are non-lattice lower semilattices.
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The object isLowerSemilattice is a method function.
The source of this document is in /build/reproducible-path/macaulay2-1.25.06+ds/M2/Macaulay2/packages/Posets.m2:5875:0.