- an object can have only 1 owner
- assignment/initialization either copies or moves the source
  * if the source doesn't implement the Copy trait, it is moved to the
    destination, i.e., ownership is transferred from the source to the
    destination
  * copy/move can also occur
    - when passing by value into a function
    - returning by value from a function
    - when pattern-matching
- at any time, an object can have
  EITHER 1 exclusive/mutable reference (&mut) to it
  OR     any number of shared references (&) to it
- we need to be aware of the scope of a reference: it starts from its
  introduction up to and including the last time it is used (i.e., scope
  of a reference is not lexical)
- if an object has any reference to it, it cannot be written to
- if an object has a exclusive reference to it, it cannot be read or written
  (the object is "frozen")

Copy trait:
- an array is Copy (i.e. implements the Copy trait) if its elements are Copy
- a tuple is Copy if its elements are Copy

Basics:
  let a = ...;
  &a (OK), &mut a (NOT OK, a is not declared mut)
  let mut a = ...;
  &a, &mut a (BOTH OK, but not at the same time)

  let a = ...;  // some type that doesn't implement Copy
  let b = a;    // a is moved to b