moebiusFunction(Poset) -- returns the Moebius function values for the unique minimal element to each element of the poset

i1 : P = poset ({a,b,c,d},{(a,b), (b,c), (b,d)})

o1 = Poset{cache => CacheTable{}                }
           GroundSet => {a, b, c, d}
           RelationMatrix => | 1 1 1 1 |
                             | 0 1 1 1 |
                             | 0 0 1 0 |
                             | 0 0 0 1 |
           Relations => {(a, b), (b, c), (b, d)}

o1 : Poset

i2 : M = moebiusFunction(P)

o2 = HashTable{a => 1 }
               b => -1
               c => 0
               d => 0

o2 : HashTable