Key points are not available for this paper at this time.
“This is a pre-copy-editing, author produced PDF of an article accepted for publication in IMA Journal of Numerical Analysis following peer review. The definitive publisher-authenticated version Vol.12, No.2 pp.135-150 is available online at: http://imajna.oxfordjournals.org/ Copyright Institute of Mathematics and its Applications.
Bruce Christianson (Wed,) studied this question.