def addone(bv):
    """Add one bit to a bit vector.

    Overflows are silently dropped.

    >>> print addone(BitVector(bitstring='1100'))
    1101

    >>> print addone(BitVector(bitstring='1111'))
    0000

    @param bv: Add one to these bits
    @type bv: BitVector
    @return: Bits with one added
    @rtype: BitVector
    """
    new = bv
    r = range(1,len(bv)+1)
    for i in r:
        index = len(bv)-i
        if 0==bv[index]:
            new[index]=1
            break
        new[index]=0
    return new