def parse_mode(mode_data):
    """ Takes in a mode string and parses out which are to be added and which
    are to be removed.
    
        >>> mode_data = "+ocn-Ct"
        >>> parse_mode(mode_data)
        ('ocn', 'Ct')
    
    """
    add = remove = ""
    directive = "+"
    for char in mode_data:
        if char in ("+", "-"):
            directive = char
        elif directive == "-":
            remove += char
        elif char == " ":
            continue
        else:
            add += char
    return (add, remove)