def platform(*types):
    """Create a platform specifier.

    >>> platform('ios', 'winphone')
    ['ios', 'winphone']
    >>> platform('ios', 'symbian')
    Traceback (most recent call last):
        ...
    ValueError: Invalid platform 'symbian'

    """
    if len(types) == 1 and types[0] == 'all':
        return 'all'
    for t in types:
        if t not in ('ios', 'android', 'winphone'):
            raise ValueError("Invalid platform '%s'" % t)
    return [t for t in types]
