Starting at the top, we depress HS-400 to open HV-400. There is a short time
lag before the valve is proved open by ZSH-400 and ZL-400 is lit.
The next input LSL-301 and LAL-301 are not activated, that is, there is liquid in
the tank. This action is reversed by the NOT and the second input is satisfied.
Next, pump start switch HS-401 is depressed, green light HL-401 is lit, and this
input is satisfied.
The fourth input HS-402, the pump stop switch, is not depressed. This action
is reversed by the NOT and the fourth input is satisfied.
The last input to MEMORY is not activated, so there is no output from
MEMORY. This action is reversed by the NOT, so this input is satisfied and
the pump starts