I got to Chapter 4: the many ways of losing your variable... Hahaha. ;_;
So you can't ever have two pointers pointing to an address in the heap, but it is OK to have many pointers (read-only) pointing to a pointer (in the stack).
Since things die very easily because they are easy to get transferred into the wrong hands which die, I wonder how one uses closure in Rust. Well, by transferring into right hands, I guess.