example.d

int abs(int x)
out (result) {
    assert(result >= 0);
} body {
    return x < 0 ? -x : x;
}

int f(int x)
in {
    assert(x % 2 == 0);
} body {
    return x / 2;
}

example.pm6

subset Nat of Int where * >= 0;
sub abs(Int $x) returns Nat {
    $x < 0 ?? -$x : $x
}

subset Even of Int where * %% 2;
sub f(Even $x) {
    return $x / 2;
}