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This week's Ruby Quiz is to write a script that finds the longest repeated
substring in a given text.

Your program will be passed some text on STDIN and is expected to print the
longest repeated substring within that text to STDOUT.

Repeated substrings may not overlap.  If more than one substring is repeated
with the same length, you may print any of them.  If there is no repeated
substring, the result is an empty string (print nothing).

Example:

	$ echo banana | ruby longest_repeated_substring.rb
	an
	
	OR
	
	$ echo banana | ruby longest_repeated_substring.rb
	na

Make sure your code runs efficiently when passed a large text.