v as List!(Nat, _) = ![]
for! 0..<10, i =>
v.push! i
assert v[0] == 0
assert v == [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
log sum v
iv: List!(Int, _) = ![1]
iv.push! 2
iv: List!(Int, 2)
v2 = ![1, 1, 1]
v2.update!((x: [{1}; _]) -> x + [1])
v3 as List!(Int, _) = ![1, 2, 3]
v3.update!(x -> x + [1])