fn main() {
    println!("Hello, world!");
    let bb = 5;
    receiver(&bb); // borrow bb immutably
    println!("BB1 {bb}");
    let mut bb = 5; // for next line to work, bb must be mutable.  So redefine bb. What happens to first bb????
    mut_receiver(&mut bb);  //to allow a mutable borrow, must state explicitly in function that the thing being loaded is mutable.
    println!("BB2 {bb}");

}

// here aa is defined to be mutable. BUT what is mutable? It is the pointer, NOT the value pointed at!!!
fn receiver(mut aa:&i32) {
    println!("{aa}");
    aa=&9;
    println!("{aa}");
}

// here we borrow with a mutable pointer. This, we can change the value that is pointed at!!
// must to be allow to do that, the function must identify the pointer as being mutable AND
// the caller of the function must agree that the thing being borrowed is mutable
fn mut_receiver(aa:&mut i32) {
    println!("{aa}");
    *aa=9;
    println!("{aa}");
}