def obter_pos_c(pos):
    """
    obter_pos_c: posicao -> str
    Esta funcao devolve a componente coluna da posicao.
    """     
    if pos[0] in (1, 4, 7):
        return 'a'
    elif pos[0] in (2, 5, 8):
        return 'b'
    elif pos[0] in (3, 6, 9):
        return 'c'