gi:16 to 4 multiplexer
https://electronics.stackexchange.com/questions/254562/block-diagram-of-161-mux-using-four-41-mux-only
g:coe 202 digital logic design
https://faculty.kfupm.edu.sa/COE/mudawar/coe202/201/index.htm
..
https://faculty.kfupm.edu.sa/COE/mudawar/coe202/lectures/index.htm
https://faculty.kfupm.edu.sa/COE/mudawar/coe202/lectures/10-FunctionalBlocks.pdf