  Optical Sommerfeld-Brillouin precursors significantly ahead of a main field
of comparable amplitude have been recently observed in an opaque medium with an
electromagnetically induced transparency window [Wei et al., Phys. Rev. Lett.
103, 093602 (2009)]. We theoretically analyze in this paper the somewhat
similar results obtained when the transparency is induced by the propagating
field itself and we establish an approximate analytic expression of the
time-delay of the main-field arrival, which fits fairly well the result
obtained by numerically solving the Maxwell-Bloch equations.
