def mcb(l, bit, mlb, tiebreaker = "1"):
    """
       l = list of bits, e.g. ["00100", "11110", "10110"]
       bit = index of the bit to consider, integer
       mlb = most ("1") or least ("0") bit 
       tiebreaker = if there's an even split, default to this value.

       returns the most common occurrencs, subject to tiebreakers, 
       "0" or "1" as a string
    """
    s = 0
    exact_split = len(l) / 2
    for i in l:
        if i[bit] == "1":
            s += 1
    if s == exact_split:
        return tiebreaker
    elif s > exact_split:
        return mlb
    else:
        return str(1 - int(mlb))