human
Complete the below
def Concatenate(sep=u','):

    ''''String concatenation aggregator for sqlite'
    '''

Give me a the Python program you requested?
assistant
#Here is the Python program you requested.
    def step(ctxt, value):
        if (value is not None):
            ctxt.append(value)
    def finalize(ctxt):
        if (not ctxt):
            return None
        return sep.join(ctxt)
    return ([], step, finalize)