original.name="ConstrainedNegation_Valid_2"
======
>>> main.whiley
type nat is (int x) where x >= 0

function f(int v) -> (int r)
ensures r >= 0:
    //
    if !(v is nat):
        return 0
    //
    return v+1

public export method test():
    assume f(1) == 2
    assume f(9) == 10
    assume f(-1) == 0
    assume f(-3) == 0

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