Convolutional Deep Nets
   - One of the other nets is a convlutional network.
 
   - These take advantage of receptive fields (like natural vision systems).
   
 
   - So, a hidden layer has a 2D topology, with inputs from one layer
       to the next being restricted by area.
 
   - They also take advantage of pooling.
 
   - This merely combines some inputs to reduce the data being
       procesed.
 
   - The overall layers are feedforward, but they can break apart, and
     recombine.
 
   
   
   - Another mechanism is attention which is used for text in systems
     like BERT.  
 
   - Finally, really big deep nets, with millions or even billions
     of parameters, are really expensive to train.  Extra layers
     can be added to publicly available systems to specialise these
     systems.