1. T : Type
2. P : T 3. T List
4. u : T 5. v : T List
6. x:T. (x filter(P;v)) (xv) & P(x)
x:T.
(x if P(u) [u / filter(P;v)] else filter(P;v) fi) (x [u / v]) & P(x)
13 steps
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