[figure: 00; 10; 20; 00; Oaxtepec Eclipse Conference, July 4-11, 1991; Ext_{A}^{s,t}(ZZ/2, ZZ/2); h_{0}; h_{1}; h_{2}; h_{2}^{2}; h_{3}; c_{0}; P h_{1}; 10; P h_{2}; h_{3}^{2}; d_{0}; h_{4}; P c_{0}; e_{0}; P^{2}h_{1}; h_{2}h_{4}; f_{0}; c_{1}; P^{2}h_{2}; 20; g; h_{2}^{2}h_{4}; h_{2}c_{1}; P d_{0}; h_{4}c_{0}; h_{2}g; i; P^{2}c_{0}]

[figure: P^{2}c_{0}; P e_{0}; P^{3}h_{1}; h_{2}^{2}g; j; P^{3}h_{2}; P g; k; 30; h_{4}^{2}; r; P^{2}d_{0}; h_{5}; n; P h_{4}c_{0}; d_{1}; q; l; P^{3}c_{0}; p; P^{2}e_{0}; P^{4}h_{1}; h_{2}h_{5}; h_{2}n; e_{0}^{2}; P j; h_{2}d_{1}; m; P^{4}h_{2}; t; P^{2}g; h_{2}^{2}h_{5}; x; e_{0}g; P k; h_{3}h_{5}; e_{1}; y; P^{3}d_{0}; h_{5}c_{0}; c_{1}g; u; P d_{0}e_{0}; P^{2}i; 40; f_{1}; g^{2}; h_{2}P k; P^{4}c_{0}; c_{2}; z; P^{3}e_{0}; P^{5}h_{1}; P h_{2}h_{5}; v; P e_{0}^{2}; P^{2}j; P m; P^{5}h_{2}; g_{2}; d_{0}r; P^{3}g; h_{4}^{3}; h_{5}d_{0}; w; P e_{0}g; P^{2}k; B_{1}; N; gj; P^{2}r; P^{4}d_{0}; h_{2}g_{2}; P h_{5}c_{0}; e_{0}r; P u; P^{2}e_{0}d_{0}; h_{3}c_{2}; h_{5}e_{0}; B_{2}; pg_{2}; P^{2}l; P^{5}c_{0}]

[figure: h_{3}c_{2}; h_{5}e_{0}; B_{2}; pg_{2}; P^{2}l; P^{5}c_{0}; h_{5}f_{0}; gk; P z; P^{4}e_{0}; P^{6}h_{1}; 50; h_{5}c_{1}; C; gr; P v; P^{2}e_{0}^{2}; P^{3}j; h_{5}g; h_{2}B_{2}; gn; e_{0}^{3}; P^{2}m; P^{6}h_{2}; D_{1}; d_{1}g; gl; P d_{0}r; P^{4}g; h_{2}h_{5}c_{1}; h_{2}C; P h_{5}d_{0}; x′′; P w; P^{2}e_{0}g; P^{3}k; G; h_{5}i; R_{1}; e_{0}^{2}g; P gj; P^{5}d_{0}; gm; P e_{0}r; P^{2}u; P^{3}e_{0}d_{0}; P^{4}i; P h_{5}e_{0}; gt; P B_{2}; d_{0}v; P^{2}g_{2}; P^{3}l; P^{6}c_{0}; Q_{2}; h_{5}j; e_{0}g^{2}; pgk; P^{2}z; P^{5}e_{0}; P^{7}h_{1}; D_{2}; P gr; P^{2}v; P^{3}e_{0}^{2}; P^{4}j; B_{21}; d_{0}w; P e_{0}^{3}; P^{3}m; P^{7}h_{2}; 60; B_{3}; B_{4}; g′′z; g_{3}; P gl; P^{2}d_{0}r; P^{5}g; 70]