-# * every node is either red or black
-# * root is black
-# * every leaf (null) is black
-# * if a node is red, then both its children are black
-# * for each node, all simple path from the node to descendant
-# leaves contain the same number of black nodes
+# * every node is either red or black
+# * root is black
+# * every leaf (null) is black
+# * if a node is red, then both its children are black
+# * for each node, all simple path from the node to descendant
+# leaves contain the same number of black nodes