This class encapsulates/makes/manipulates objects that represent nodes in a
tree structure. The tree structure is not an object itself, but is emergent
from the linkages you create between nodes. This class provides the methods
for making linkages that can be used to build up a tree, while preventing you
from ever making any kinds of linkages that are not allowed in a tree (such as
having a node be its own mother or ancestor, or having a node have two
mothers).
