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
which are not allowed in a tree (such as having a node be its own
mother or ancestor, or having a node have two mothers).