  We discuss a simple model for the formation of a supernova spectral line by
resonance scattering in the presence of external illumination of the
line-forming region by light from circumstellar interaction (toplighting). The
simple model provides a clear understanding of the most conspicuous toplighting
effect: a rescaling or, as we prefer, a ``muting'' of the line profile relative
to the continuum. This effect would be present in more realistic models, but
would be harder to isolate. An analytic expression for a muting factor for a
P-Cygni line is derived that depends on the ratio E of the toplighting specific
intensity to the specific intensity from the supernova photosphere. If E<1, the
line profile is reduced in scale or ``muted''. If E=1, the line profile
vanishes altogether. If E>1, the line profile flips vertically: then having an
absorption component near the observer-frame line center wavelength and a
blueshifted emission component.
