def css_prefixer(text):
    """ Prefix Phosphor CSS with .flx-Widget so it does not interfere
    with other Phosphor JS on the page.
    """
    lines = []
    for line in text.splitlines():
        if '.p-' in line:
            p_selectors = [p.strip() for p in line.strip(' {').split(',')]
            p_selectors = [p for p in p_selectors if p.startswith('.p-')]
            if p_selectors:
                if line.startswith('.p-') and len(p_selectors) == 1:
                    if p_selectors[0] == '.p-Widget':
                        line = line.replace('.p-Widget', '.flx-Widget')
                    else:
                        line = '.flx-Widget ' + line
                else:
                    raise ValueError('Cannot yet parse multiple selectors per line.')
        lines.append(line)
    return '\n'.join(lines)