C. Olds, A. Lax, and G. Davidoff. Anneli Lax New Mathematical Library Mathematical Association of America, Washington, DC, (2000)Appendix I by Peter D. Lax.
L. Sabalka, and D. Savchuk. (2010)cite arxiv:1007.1998
Comment: 18 pages, 5 figures. Revised version has many corrections and
revisions, including more explanation of definitions and proofs.