literature in the last couple of decades [@Aich91; @Dani02]. However, the density dependence of the symmetry energy, from the Coulomb barrier to the deconfinement of nuclear matter, is a hot topic in the present era. At sub-saturation densities, the density dependence of the symmetry energy is studied by observables such as the neutron-to-proton ratio, isotopic and isobaric scaling, isospin diffusion, isospin fractionation and/or distillation, and isospin migration. [@Li97; @Zhan08; @Sun10; @Kuma11]. Recently, the MSU group [@Zhan08] claimed the softness of the symmetry energy at sub-saturation densities by using the double neutron-to-proton ratio and isospin diffusion from two isotopic systems, $^{112}$Sn + $^{112}$Sn and $^{124}$Sn + $^{124}$Sn at E = 50 MeV/nucleon. In another study, again soft symmetry energy was claimed by using the isospin diffusion for the same set of reactions, but at E = 35 MeV/nucleon [@Sun10]. In a recent study, soft symmetry energy is also favored for the same set of reactions at E = 50 MeV/nucleon by using the neutron-to-proton ratio [@Kuma11]. In all the studies, the problem of sub-saturation density dependence of the symmetry energy seems to be addressed to some extent; however, the uncertainties are still large enough to justify the large amount of work 