human
Complete this python program:
def Concatenate(sep=u','):
assistant
#Here is the Python program you requested.

    """String concatenation aggregator for sqlite
    """
    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)