<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def create (raw_properties = []):
 """ Creates a new 'PropertySet' instance for the given raw properties, or returns an already existing one. """

    assert (is_iterable_typed(raw_properties, property.Property)
            or is_iterable_typed(raw_properties, basestring))
    # FIXME: propagate to callers.
    if len(raw_properties) > 0 and isinstance(raw_properties[0], property.Property):
        x = raw_properties
    else:
        x = [property.create_from_string(ps) for ps in raw_properties]

    # These two lines of code are optimized to the current state
    # of the Property class. Since this function acts as the caching
    # frontend to the PropertySet class modifying these two lines
    # could have a severe performance penalty. Be careful.
    # It would be faster to sort by p.id, but some projects may rely
    # on the fact that the properties are ordered alphabetically. So,
    # we maintain alphabetical sorting so as to maintain backward compatibility.
    x = sorted(set(x), key=lambda p: (p.feature.name, p.value, p.condition))
    key = tuple(p.id for p in x)

    if key not in __cache:
        __cache [key] = PropertySet(x)

    return __cache [key]