i = !0
i.update! _ -> 1
i.update! i -> i * 3
assert i == 3
i.inc!()
assert i == 4
i.dec!()
assert i == 3
assert i.abs() == 3
ss = !"abc"
ss.push! "d"
assert ss == "abcd"
discard ss.pop!()
assert ss == "abc"
discard ss.remove! 0
assert ss == "bc"
assert ss.capitalize() == "Bc"
write1!(ref! x: Nat!) =
x.update! _ -> 1
n = !0
write1! n
assert n == 1