This work was supported by Fundação para a Ciência e Tecnologia (FCT) under project BULET/33980/99, cofinanced by the European Union under program FEDER. MACT acknowledges the financial support of FCT under Grant SFRH/BPD/3533/ 2000. Suggestions and comments made by B. Grisogono and two anonymous referees are also gratefully acknowledged.
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