Mutation and assignment

Add mut when the binding itself must change. Assignment is a statement and produces no value. Compound forms such as +=, *=, and <<= read and write the target once.

moon does not convert the assigned value implicitly, so the right side must already have the target's type.

fn main() {
  let mut score = 5
  score += 1
  score *= 4
  println(score)
}