def prettify(element, *, default_category):
    """Return element properties formatted as list. The type of the element
    (recurrent or standard) is inferred by the presence of the 'frequency'
    property.
    If the element's 'category' property is None, use 'default_category'.

    :type element: dict
    :type default_category: str
    """
    recurrent = 'frequency' in element
    if recurrent:
        properties = 'name', 'value', 'frequency', 'start', 'end', 'category'
    else:
        properties = 'name', 'value', 'date', 'category'
    longest_property_length = 0
    for p in properties:
        if len(p) > longest_property_length:
            longest_property_length = len(p)
    if element['category'] is None:
        element['category'] = default_category
    lines = []
    for p in properties:
        lines.append('{}: {}'.format(p.capitalize().ljust(
            longest_property_length), str(element[p]).title()))
    return '\n'.join(lines)
