Received for publication 18 Dec. 1986. Funding was provided by Colorado State Univ. Agricultural Experiment Station (Project 621). F.D.M. thanks the host department at the Univ. of British Columbia for facility usage and computing support during his sabbatical leave, and Phillip Chapm an, Statistics D epartm ent, Colorado State Univ. M ention of a tradem ark, proprietary product, or vendor does not constitute guarantee or warranty of the product by the authors and does not imply its approval to the exclusion of other products or vendors that also may be suitable. The cost of publishing this paper was defrayed in part by the payment of page charges. Under postal regulations, this paper therefore must be hereby marked advertisement solely to indicate this fact.