code: def create(cls, propertyfile, allow_unknown):
          
          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)
docs: 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