  In their recent paper [Phys. Rev. A 71, 033622], B. T. Seaman et al. studied
Bloch states of the condensate wave function in a Kronig-Penney potential and
calculated the band structure. They argued that the effective mass is always
positive when a swallow-tail energy loop is present in the band structure. In
this comment, we reexamine their argument by actually calculating the effective
mass. It is found that there exists a region where the effective mass is
negative even when a swallow-tail is present. Based on this fact, we discuss
the interpretation of swallow-tails in terms of superfluidity.
