Introduced singularity functions u_k(t) where u_0(t) = delta(t), u_{-1}(t) = u(t), and
u_{k+r}(t) = u_k(t) * u_r(t). 
In particular, u_1(t) is the impulse response for the system y(t) = d(x(t))/dt and is
called the unit doublet.
Defined unit step response.
Started an example on correcting effect of echoes on an acoustic signal (2.64).