Nuntar wrote:
What you need to do is

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×build a cage out of tarstuff, so that the adder stays north of the long pressure plate, and hits the 1-tile pressure plate repeatedly.

It helps to remember that adders only eat the uncuttable tiles of tarstuff: edges for mud, corners for tar.

So the easiest way to set things up is

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×a U-shaped basin of tar, so that when the adder is on the inside, the only tiles it has access to are edges.