class FibonacciSequence
	**Prints the nth fibonacci number**
	
	on start
		
		args := program arguments
		
		if args empty
			print .fibonacci(8)
		
		else
			
			try
				print .fibonacci(integer.parse(args[0]))
			
			catch FormatException
				print to Console.error made !, "Input must be an integer"
				exit program with error code
			
			catch OverflowException
				print to Console.error made !, "Number too large"
				exit program with error code
	
	define fibonacci(n as integer) as integer is shared
		**Returns the nth fibonacci number**
		
		test
			assert fibonacci(0) = 0
			assert fibonacci(1) = 1
			assert fibonacci(2) = 1
			assert fibonacci(3) = 2
			assert fibonacci(4) = 3
			assert fibonacci(5) = 5
			assert fibonacci(6) = 8
			assert fibonacci(7) = 13
			assert fibonacci(8) = 21

		
		body
			a, b := 0, 1
		
			for n
				a, b := b, a + b
		
			return a