In Part 5 of the ZkSNARK introduction, under the “Reduction to a QAP” paragraph reads:

Concretely, as w1,w2,w4 are, respectively, the left, right and output wire of g1; we define L1=R2=O4=2−X, as the polynomial 2−X is one on the point 1 corresponding to g1 and zero on the point 2 corresponding to g2.

Note that w1and w3 are both right inputs of g2. Therefore, we define similarly L4=R1=R3=O5=X−1 – as X−1 is one on the target point 2 corresponding to g2 and zero on the other target point.

I don’t understand where the 2-X and X-1 components are derived from. Could anyone please explain this concept?