The Hasse diagram of a poset is a Digraph with vertices given by the ground set of $P$ and which has the direct edge $(a,b)$ if and only if $a < b$ in $P$ and there exists no $c$ such that $a < c < b$.

This method renames the vertices with integers $0, 1, \ldots$ corresponding to the index of the vertices in the GroundSet.
The object hasseDiagram is a method function.