def indexToSymbol(index):
    """
    returns the nucleotide symbol for an index in the alphabetic order
    """
    if index == 0: return 'A'
    elif index == 1: return 'C'
    elif index == 2: return 'G'
    elif index == 3: return 'T'
    else:
        print('number should be  0 to 3, it is ' + str(index))
        return 'X'