\chapter{{\tt ETree}: Elimination and Front Trees}
\label{chapter:ETree}
\par
The {\tt ETree} object is used to model an elimination tree or a
front tree for a sparse factorization with symmetric structure.
The tree is defined over a set of vertices in a graph --- the graph
can be unit weight or non-unit weight.
A ``node'' in the tree can be a single vertex (in the context of 
an elimination tree) or a group of vertices (as for a front tree).
\par
The tree information is stored as a {\tt Tree} object.
In addition there are three {\tt IV} objects.
One stores the total size of the nodes in the fronts,
one stores the size of the boundaries of the fronts,
and one stores the map from the vertices to the fronts.
\par
There is a great deal of functionality embodied into the {\tt ETree} 
object.
Given an elimination tree or a front tree, one can extract the
permutation vectors (for the fronts or the vertices), extract a
multisector based on several criteria, compress the front tree in
several ways, justify the tree (order children of a node in
meaningful ways), evaluate metric vectors on the tree (heights,
depths, subtree accumulators).
\par
The front tree we obtain from a low-fill matrix ordering is usually
not the front tree that drives the factorization.
We provide three methods that transform the former into the latter.
One method merges the fronts together in a
way that adds logical zeros to their structure.
One method splits large fronts into smaller fronts.
One method combines these two functionalities.