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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 addinterval(instr, add, interval):
    '''
        adds string every n character. returns string
    '''
    if not isinstance(instr, str):
        instr = str(instr)

    return add.join(
        instr[i:i+interval]
        for i in xrange(0,len(instr),interval))