def convert(coord):
    """Converts coordinate as displayed coordinates to digit tuple eg: J9 ->
    (9,9); checks for possible human errors converts coordinate as digit tuple
    to displayed coordinates eg: (0,0) -> A0; should not have any errors.
    """
    A2D = dict(zip('ABCDEFGHIJ', range(10)))
    D2A = dict(zip(range(10), 'ABCDEFGHIJ'))
    if isinstance(coord, str):
        try:
            coord = A2D[coord[0]], int(coord[1])
            if 0 <= coord[1] < 10:
                return coord
            else:
                return None
        except KeyError:
            return None
    else:
        coord = D2A[coord[0]] + str(coord[1])
        return coord
