Explain why am getting this error

#![allow(warnings)] fn main() { let mut a = 4; let mut b = &mut a; let mut c : &mut &mut u32 = &mut b; let mut d = 44; let mut e = &mut d; let mut f : &mut &mut u32 = &mut e; let mut g : &mut &mut &mut u32 = &mut f; println!("{}",f); let mut h = 444; e = &mut h; g = &mut c; println!("{}",g); } (Playground) Errors: Compiling playground v0.0.1 (/playground) error[E0506]: cannot assign to `e` because it is borrowed --> src/main.rs:23:1 | 14 | let mut f : &mut &m...