The full adder: a carry comes in, a carry goes out

Add a carry from the place below. Three trits sum to -3..3, still one sum trit and one carry trit: 27 cases, every one shown.

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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