def prevCol(column):
    """Return the column letter that comes before column.

    Returns '' if column 'A'."""
    return {'': '', 'A': '', 'B': 'A', 'C': 'B', 'D': 'C',
            'E': 'D', 'F': 'E', 'G': 'F', 'H': 'G'}[column]