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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def create(cls, propertyfile, allow_unknown):
 """ Create a Property instance by attempting to parse the given property file. @param propertyfile: A file name of a property file @param allow_unknown: Whether to accept unknown properties """

        with open(propertyfile) as f:
            content = f.read().strip()

        # parse content for known properties
        is_svcomp = False
        known_properties = []
        only_known_svcomp_property = True

        if content == 'OBSERVER AUTOMATON' or content == 'SATISFIABLE':
            known_properties = [_PROPERTY_NAMES[content]]

        elif content.startswith('CHECK'):
            is_svcomp = True
            for line in filter(None, content.splitlines()):
                if content.startswith('CHECK'):
                    # SV-COMP property, either a well-known one or a new one
                    props_in_line = [
                        prop for (substring, prop) in _PROPERTY_NAMES.items() if substring in line]
                    if len(props_in_line) == 1:
                        known_properties.append(props_in_line[0])
                    else:
                        only_known_svcomp_property = False
                else:
                    # not actually an SV-COMP property file
                    is_svcomp = False
                    known_properties = []
                    break

        # check if some known property content was found
        subproperties = None
        if only_known_svcomp_property and len(known_properties) == 1:
            is_well_known = True
            name = known_properties[0]

        elif only_known_svcomp_property and set(known_properties) == _MEMSAFETY_SUBPROPERTIES:
            is_well_known = True
            name = _PROP_MEMSAFETY
            subproperties = list(known_properties)

        else:
            if not allow_unknown:
                raise BenchExecException(
                    'File "{0}" does not contain a known property.'.format(propertyfile))
            is_well_known = False
            name = os.path.splitext(os.path.basename(propertyfile))[0]

        return cls(propertyfile, is_well_known, is_svcomp, name, subproperties)