The Tower of Hanoi (also called the Tower of Brahma or Lucas' Tower and sometimes pluralized) is a mathematical game or puzzle. It consists of three rods and a number of disks of different sizes, which can slide onto any rod. The puzzle starts with the disks in a neat stack in ascending order of size on one rod, the smallest at the top, thus making a conical shape.

The objective of the puzzle is to move the entire stack to another rod, obeying the following simple rules:

• Only one disk can be moved at a time.
• Each move consists of taking the upper disk from one of the
stacks and placing it on top of another stack. • No disk may be placed on top of a smaller disk.

With 3 disks, the puzzle can be solved in 7 moves. The minimal number of moves required to solve a Tower of Hanoi puzzle is 2^n-1, where n is the number of disks.

Taken from

~~~{ 'Num 'From 'To 'Via } [ var ] a:for-each   :set     !Via !To !From !Num ; :display @To @From 'Move_a_ring_from_%n_to_%n\n s:format s:put ;   :hanoi (num,from,to,via-)   set @Num n:-zero?   [ @Num @From @To @Via     @Num n:dec @From @Via @To hanoi set display     @Num n:dec @Via @To @From hanoi ] if ;   #3 #1 #3 #2 hanoi nl ~~~