The half adder: two trits in, a sum and a carry out

Pick two trits. The half adder writes their sum as a sum trit plus three times a carry trit; all nine cases below.

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Where the numbers come from

gHashTag/t27 specs/ternary/trits.t27 (t27#8979): every value on the page is its fn bodies, compiled to wasm (logic.js), with 18 tests and 4108 runtime asserts
D. Knuth, The Art of Computer Programming, vol. 2, section 4.1: balanced ternary