+#
+# Nodes and edges are added to the POSet.
+#
+# ~~~
+# var pos = new POSet[String]
+# pos.add_edge("A", "B") # add A->B
+# pos.add_edge("B", "C") # add B->C
+# pos.add_node("D") # add unconnected node "D"
+#
+# # A -> B -> C D
+#
+# assert pos.has_edge("A", "B") == true # direct
+# ~~~
+#
+# Since a poset is transitive, direct and indirect edges are considered by default.
+# Direct edges (transitive-reduction) can also be considered independently.
+#
+# ~~~
+# assert pos.has_edge("A", "C") == true # indirect
+# assert pos.has_edge("A", "D") == false # no edge
+# assert pos.has_edge("B", "A") == false # edges are directed
+#
+# assert pos.has_direct_edge("A", "B") == true # direct
+# assert pos.has_direct_edge("A", "C") == false # indirect
+# ~~~
+#
+# POSet are dynamic.
+# It means that the transitivity is updated while new nodes and edges are added.
+# The transitive-reduction (*direct edges*)) is also updated,
+# so adding new edges can make some direct edge to disappear.
+#
+# ~~~
+# pos.add_edge("A","D")
+# pos.add_edge("D","B")
+# pos.add_edge("A","E")
+# pos.add_edge("E","C")
+#
+# # A -> D -> B
+# # | |
+# # v v
+# # E ------> C
+#
+# assert pos.has_edge("D", "C") == true # new indirect edge
+# assert pos.has_edge("A", "B") == true # still an edge
+# assert pos.has_direct_edge("A", "B") == false # but no-more a direct one
+# ~~~
+#
+# Thanks to the `[]` method, elements can be considered relatively to the poset.
+# SEE `POSetElement`
+class POSet[E]