Elimination tree of a matrix.
p = etree(A) returns an elimination tree for the square
symmetric matrix whose upper triangle is that of A.
p(j) is the parent of column j in the tree,
or 0 if j is a root.
p = etree(A,'col') returns the elimination tree of A'*A.
p = etree(A,'sym') is the same as p = etree(A).
[p,q] = etree(...) also returns a postorder permutation q on the tree.
See also TREELAYOUT , TREEPLOT , ETREEPLOT .