[D] How does SHAP work for seq2seq models?

Suppose we have a seq2seq function s(x_1,x_2,x_3) -> y_1 y_2 y_3, how exactly is SHAP defined in this case? Specifically, what exactly is the function f defined in Theorem 1 in the paper? https://arxiv.org/pdf/1705.07874.pdf

I am guessing we first fix a y_i, i={1,2,3} and we define f(x) = 1 if y_i is in the output of s(x), 0 otherwise, where x = x_1, x_2, x_3. Is that correct?

Thanks!

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