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Link is a project by d4l3k that is designed to facilitate complex mechanisms in Minecraft. It is a sign based system. The signs are called "Gates".
This is an example of a Toggle (which toggles between values), outputting to redstone.
This runs a server command when the Toggle is toggled to 1.
Each gate consists of two things. A name and sometimes extra lines for configuration.
The first line of a gate when its being constructed is the gate ID. Ex: [Constant]
Some gates this is all that is needed, but for some gates like the Constant Value Gate, it needs extra data.