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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def common_prefix(s1, s2):
 		""" 		Return the common prefix of two lines. 		"""

		index = min(len(s1), len(s2))
		while s1[:index] != s2[:index]:
			index -= 1
		return s1[:index]