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Arithmetic Combinator : 
Iron and copper sum is 30, the lamp is set to < 5. Then why the lamp is turn on ?
Decider Combinator :
Iron is less than copper, so it's supposed to give 1 green circuit signal output.
Lamp is set to turn on if the green circuit signal is 1, then why it's still turn off ?
That's how i interpret how combinator work.
For hours i can't get it to work basic function.
Just before i post the question to forum, i thought,..maybe...the input/output side maybe are dedicated one, unlike smart inserter.
TLDR: Combinator orientation matter. I hope this can be shown graphically more easily. Really confusing for noob.
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