Sequences of integers generated in the Collatz problem. For example, for a starting number of 7, the sequence is 7, 22, 11,
34, 17, 52, 26, 13, 40, 20, 10, 5, 16, 8, 4, 2, 1, 4, 2, 1, .... Such sequences are
called hailstone sequences because the values typically rise and fall, somewhat analogously
to a hailstone inside a cloud.
While a hailstone eventually becomes so heavy that it falls to ground, every starting positive integer ever tested has produced a hailstone
sequence that eventually drops down to the number 1 and then "bounces"
into the small loop 4, 2, 1, ....
