fn foo(a mut []int) {
a[0] = 7
a << 4
}
fn test_mut() {
mut a := [1,2,3]
foo(mut a)
assert a.len == 4
assert a[0] == 7
assert a[3] == 4
n := 1
mut b := &n
*b = 10
//mut b := mut a
//b = 10