A. Chéritat. (2014)cite arxiv:1410.4417Comment: 16 pages, 7 figures. This version has the following changes: Added computer generated images of the key positions S1 and S2. Corrected several minor mistakes. Corrected the proof of the main proposition (I had forgotten to ensure that the top and bottom curves remain embedded during the homotopy) and slightly changed the statement of Lemma 3 to adapt.