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Multiplexing

A multiplexer is a switch that channels one of the inputs to the output. A 2-to-1 multiplexer outputs AS + BS, where A and B are the inputs, and S is the selector that controls the switch.

From De Morgan's laws:

AS + BS

= AS + B + S

= AS(B + S)

That suggests two NAND gates, an OR gate, and an inverter arranged as follows.

The code below is based on that schematic.

muxLeft.t
ih 0
ih 4
ih -4
notLeft -4 1
notRight -1 1
notRight 4 1
jr 1
o 2
ih -2
ih 3
nand 0 7

in l -6..-3 0
in s -2..1 0
in r 2..5 0
out o -2..1 13

Here are the results for all input combinations:

The selector (the center input) is 0 in the upper-row and 1 in the lower-row.

As suggested by its name, muxLeft outputs the left-input when the selector is 1. A horizontally-reflected version, muxRight, outputs the right-input when the selector is 1.