Here, the output signal equals the input signal flipped about the time origin.
Again, such systems are difficult to build, but the notion of time reversal occurs frequently in communications systems.
Mentioned earlier was the issue of whether the ordering of systems mattered. In other words, if we have two systems in cascade, does the output depend on which comes first? Determine if the ordering matters for the cascade of an amplifier and a delay and for the cascade of a time-reversal system and a delay.