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
