  It says in the report$^1$ by Wang et al. that a negative group velocity
$u=-c/310$ is obtained and that a pulse advancement shift 62-ns is measured.
The authors claim that the negative group velocity is associated with
superluminal light propagation and that the pulse advancement is not at odds
with causality or special relativity. However, it is shown here that their
conclusions above are not true. Furthermore, I give some suggestion concerning
a re-definition of group-velocity and a new explanation in special relativity
of causality.
