Here is a new application of THAT 1240.
It is easy to make an adder and a subtractor using the latter because all precision resistors are included in the THAT1240 housing which reduces the size of the PCB. The assembly is reduced to its simplest form with a few connection and decoupling capacitors.
The generation of L-R & L + R composite signals is often found in phase correlators, in Dolby Prologic decoders, in FM for the multiplex signal, in voice cancellors, in telecommunications but also in measurement devices.
 For stereo sound recording enthusiasts, it is the basis of a matrix (Middle & Sides), have a look on the net because this technique also uses the sum and the difference (M + S & M-S)
In an FM tuner, following the demodulation of the signals, we recover L + R & L-R. If we add up, we have L + R + L-R = 2L. Let's differentiate, we have L + R-L - (- R) = 2R. We therefore recovered our 2L & 2R stereo signal. Clearly it's L & R increased by 6dB
.By a cabling trick, it is possible with an inverter and a single That 1240 to combine the 2 functions (see new diagram) 
It can be the basic setup for a lot of experimentation. It's your turn !