One of the simplest exercises from [George Bain’s reference book](http://www.worldcat.org/oclc/41153804), p. [28](https://books.google.fr/books?id=tSRgDwAAQBAJ&pg=PA28) plate B stage 5. Redrawn in Adobe Illustrator in 2008, converted to 3d curve in Blender in 2009.