  Very recently Girardeau and Minguzzi [arXiv:0807.3366v2, Phys. Rev. A 79,
033610 (2009)] have studied an impurity in a one-dimensional gas of hard-core
bosons. In particular they deal with the general case where the mass of the
impurity is different from the mass of the bosons and the impurity-boson
interaction is not necessarily infinitely repulsive. We show that one of their
initial step is erroneous, contradicting both physical intuition and known
exact results. Their results in the general case apply only actually when the
mass of the impurity is infinite.
