def calc_tr(tr, new_highs, new_lows, new_closes):
    """
    INPUTS:
        tr (Series) - A copy of last record's True Range value.
        new_highs (Series) - New values, prepended with last high price
        new_lows (Series) - New values, prepended with last low price
        new_closes (Series) - New values, prepended with last close price
    OUTPUT:
        True Range as a pandas Series
    """
    for n,index in enumerate(new_highs[1:].index, start=1):
        tr[index] = max(  [abs(new_highs[n] -   new_lows[n]  ),
                           abs(new_highs[n] - new_closes[n-1]),
                           abs( new_lows[n] - new_closes[n-1])]  )
    return tr