1. *x1 = 3.14, *x2 = 3.14, x1 == x2: true
2. a = 13, b = 13, p points at b: false
3. before *q = *p: *p = 13, *q = 3, p == q: false
   after  *q = *p: *p = 13, *q = 13, p == q: false, c = 13
4. after  q = p:   *p = 13, *q = 13, p == q: true, c = 3
5. after *q = 99:  a = 99, *p = 99, c = 3
