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 P = \left\{ {p(y|x)\left| \begin{array}{l} \forall {f_i}:\sum\limits_{(x,y)} {p(y|x)\bar p(x){f_i}(x,y)} = \sum\limits_{(x,y)} {\bar p(x,y){f_i}(x,y)} \\ \forall x:\sum\limits_y {p(y|x) = 1} \end{array} \right.} \right\}