implement the F,H,M triangles #34368
Description
as useful invariants of posets and simplicial complexes
CC: @mantepse @tscrim @jhpalmieri
Component: combinatorics
Author: Frédéric Chapoton
Branch/Commit: aa6105d
Reviewer: Travis Scrimshaw, John Palmieri
Issue created by migration from https://trac.sagemath.org/ticket/34368