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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 add_colons(s):
 """Add colons after every second digit. This function is used in functions to prettify serials. 'te:st:st:ri:ng' """

    return ':'.join([s[i:i + 2] for i in range(0, len(s), 2)])