[1] Univ Buenos Aires, Dept Computac, FCEyN, RA-1053 Buenos Aires, DF, Argentina
[2] Univ Buenos Aires, Dept Matemat, FCEyN, RA-1053 Buenos Aires, DF, Argentina
[3] Hasselt Univ, Theoret Comp Sci Grp, Hasselt, Belgium
[4] Univ Limburg, NL-6200 MD Maastricht, Netherlands
[5] Consejo Nacl Invest Cient & Tecn, RA-1033 Buenos Aires, DF, Argentina