  The mechanisms leading to a seemingly superluminal propagation of light in
dispersive media are examined. The anomalous dispersion near an absorption
line, involved in the first experiments displaying negative group velocity
propagation, is shown to be not only simpler but also more efficient than the
so-called transparent dispersion used in the later experiments of Wang et al.
(Nature (London) 406, 277 (2000)) and of Dogariu et al. (Phys.Rev.A 63, 053806
(2001)). The modest pulse advances observed by these authors can in particular
be reproduced with a better overall transmittance of the medium and the
significant advances attained with a single-line arrangement would probably be
unattainable with their gain-doublet arrangement.
