/usr/bin/perl -w /home/lanx/perl/IPL/poset.pl 
---------- Input
{
  Cat => { Dog => 1 },
  Dog => { Donkey => 1 },
  Donkey => {},
  Rooster => { Cat => 1 },
}
---------- Phase: 0
["Donkey"]
{ Cat => { Dog => 1 }, Dog => {}, Rooster => { Cat => 1 } }
---------- Phase: 1
["Dog"]
{ Cat => {}, Rooster => { Cat => 1 } }
---------- Phase: 2
["Cat"]
{ Rooster => {} }
---------- Phase: 3
["Rooster"]
{}
---------- Result
["Donkey", "Dog", "Cat", "Rooster"]