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by Christer Nilsson

You have a starting point and a target, say 2 and 9.
 
You have a set of three operations:

	double
	halve    (Odd numbers cannot be halved.)
	add_two

Problem: Move from the starting point to the target, minimizing the number of
operations.

	Examples:
	
	solve(2,9)  # => [2,4,8,16,18,9]
	solve(9,2)  # => [9,18,20,10,12,6,8,4,2]