https://www.geeksforgeeks.org/digital-logic-code-converters-bcd8421-tofrom-excess-3/

https://www2.southeastern.edu/Academics/Faculty/kyang/2018/Spring/CMPS375/ClassNotes/CMPS375ClassNotesChap03.pdf

https://www.seas.upenn.edu/~cit595/cit595s10/lectures/combcircuits.pdf

https://people.cs.clemson.edu/~yfeaste/855Assignments/presentations/Team6-Combinational Logic Circuit.pdf

http://www.ee.surrey.ac.uk/Projects/CAL/digital-logic/combindex.html