Much cleaner than my previous attempt.
Based on the following schematics from Wikipedia:
https://commons.m.wikimedia.org/wiki/File:Partial_Full-Adder.svg
https://commons.m.wikimedia.org/wiki/File:4-bit_carry_lookahead_adder.svg
The switches modify the operands
The logic source is the add/subtract instruction, where 0 = add, 1 = subtract. The decimal point on the second operand will light when the instruction is to subtract.
The decimal point on the result represents whether there is a carry.
