(under the class definition) Subject matter having either (1) one energy source and first and second light sources and means to supply energy from the energy source to the second light source in response to failure of the first light source; or (2) first and second energy sources and one light source and means to cause the second energy source to supply energy to the light source in response to failure of the first energy source; or (3) first and second energy sources and first and second light sources and means to cause the second energy source to supply energy to the second light source in response to failure of either the first energy source or the first light source.
