Elimination tree
p = etree(A)
p = etree(A,'col')
p = etree(A,'sym')
[p,q] = etree(...)
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 of the tree.