human
What is a the Python program you requested given the following:
Complete the below
def SortedConcatenate(sep=u','):

    ''''String concatenation aggregator for sqlite, sorted by supplied index'
    '''
assistant
#Here is the Python program you requested.
    def step(ctxt, ndx, value):
        if (value is not None):
            ctxt[ndx] = value
    def finalize(ctxt):
        if (len(ctxt) == 0):
            return None
        return sep.join(map(ctxt.get, sorted(ctxt.iterkeys())))
    return ({}, step, finalize)