1. T : Type
2. P : T 3. T List
4. u : T 5. v : T List
6. L1:T List. L1v filter(P;L1) filter(P;v)
L1:T List.
L1 [u / v] filter(P;L1) if P(u) [u / filter(P;v)] else filter(P;v) fi
5 steps
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