This module provides methods for working with adjacency lists.  The
adjacency list model is a very common way of representing a tree
structure.  In this model each row in a table has a prent ID column
that references the primary key of another row in the same table.
Because of this the primary key must only be one column and is usually
some sort of integer.  The row with a parent ID of 0 is the root node
and is usually the parent of all other rows.  Although, there is no
limitation in this module that would stop you from having multiple
root nodes.