text: """STIX 2.1 Relationship Objects."""
  
  from collections import OrderedDict
  
  from ..core import STIXRelationshipObject
  from ..properties import (
      BooleanProperty, IDProperty, IntegerProperty, ListProperty,
      ReferenceProperty, StringProperty, TimestampProperty, TypeProperty,
  )
  from ..utils import NOW
  from .common import ExternalReference, GranularMarking
  
  
  class Relationship(STIXRelationshipObject):
      # TODO: Add link
      """For more detailed information on this object's properties, see
      `the STIX 2.1 specification <link here>`__.
      """
  
      _invalid_source_target_types = ['bundle', 'language-content', 'marking-definition', 'relationship', 'sighting']
  
      _type = 'relationship'
      _properties = OrderedDict([
          ('type', TypeProperty(_type)),
          ('spec_version', StringProperty(fixed='2.1')),
          ('id', IDProperty(_type, spec_version='2.1')),
          ('created_by_ref', ReferenceProperty(valid_types='identity', spec_version='2.1')),
          ('created', TimestampProperty(default=lambda: NOW, precision='millisecond')),
          ('modified', TimestampProperty(default=lambda: NOW, precision='millisecond')),
          ('relationship_type', StringProperty(required=True)),
          ('description', StringProperty()),
          ('source_ref', ReferenceProperty(invalid_types=_invalid_source_target_types, spec_version='2.1', required=True)),
          ('target_ref', ReferenceProperty(invalid_types=_invalid_source_target_types, spec_version='2.1', required=True)),
          ('start_time', TimestampProperty()),
          ('stop_time', TimestampProperty()),
          ('revoked', BooleanProperty(default=lambda: False)),
          ('labels', ListProperty(StringProperty)),
          ('confidence', IntegerProperty()),
          ('lang', StringProperty()),
          ('external_references', ListProperty(ExternalReference)),
          ('object_marking_refs', ListProperty(ReferenceProperty(valid_types='marking-definition', spec_version='2.1'))),
          ('granular_markings', ListProperty(GranularMarking)),
      ])
  
      # Explicitly define the first three kwargs to make readable Relationship declarations.
      def __init__(
          self, source_ref=None, relationship_type=None,
          target_ref=None, **kwargs
      ):
          # Allow (source_ref, relationship_type, target_ref) as positional args.
          if source_ref and not kwargs.get('source_ref'):
              kwargs['source_ref'] = source_ref
          if relationship_type and not kwargs.get('relationship_type'):
              kwargs['relationship_type'] = relationship_type
          if target_ref and not kwargs.get('target_ref'):
              kwargs['target_ref'] = target_ref
  
          super(Relationship, self).__init__(**kwargs)
  
      def _check_object_constraints(self):
          super(self.__class__, self)._check_object_constraints()
  
          start_time = self.get('start_time')
          stop_time = self.get('stop_time')
  
          if start_time and stop_time and stop_time <= start_time:
              msg = "{0.id} 'stop_time' must be later than 'start_time'"
              raise ValueError(msg.format(self))
  
  
  class Sighting(STIXRelationshipObject):
      # TODO: Add link
      """For more detailed information on this object's properties, see
      `the STIX 2.1 specification <link here>`__.
      """
  
      _type = 'sighting'
      _properties = OrderedDict([
          ('type', TypeProperty(_type)),
          ('spec_version', StringProperty(fixed='2.1')),
          ('id', IDProperty(_type, spec_version='2.1')),
          ('created_by_ref', ReferenceProperty(valid_types='identity', spec_version='2.1')),
          ('created', TimestampProperty(default=lambda: NOW, precision='millisecond')),
          ('modified', TimestampProperty(default=lambda: NOW, precision='millisecond')),
          ('description', StringProperty()),
          ('first_seen', TimestampProperty()),
          ('last_seen', TimestampProperty()),
          ('count', IntegerProperty(min=0, max=999999999)),
          ('sighting_of_ref', ReferenceProperty(valid_types="only_SDO", spec_version='2.1', required=True)),
          ('observed_data_refs', ListProperty(ReferenceProperty(valid_types='observed-data', spec_version='2.1'))),
          ('where_sighted_refs', ListProperty(ReferenceProperty(valid_types='identity', spec_version='2.1'))),
          ('summary', BooleanProperty()),
          ('revoked', BooleanProperty(default=lambda: False)),
          ('labels', ListProperty(StringProperty)),
          ('confidence', IntegerProperty()),
          ('lang', StringProperty()),
          ('external_references', ListProperty(ExternalReference)),
          ('object_marking_refs', ListProperty(ReferenceProperty(valid_types='marking-definition', spec_version='2.1'))),
          ('granular_markings', ListProperty(GranularMarking)),
      ])
  
      # Explicitly define the first kwargs to make readable Sighting declarations.
      def __init__(self, sighting_of_ref=None, **kwargs):
          # Allow sighting_of_ref as a positional arg.
          if sighting_of_ref and not kwargs.get('sighting_of_ref'):
              kwargs['sighting_of_ref'] = sighting_of_ref
  
          super(Sighting, self).__init__(**kwargs)
  
      def _check_object_constraints(self):
          super(self.__class__, self)._check_object_constraints()
  
          first_seen = self.get('first_seen')
          last_seen = self.get('last_seen')
  
          if first_seen and last_seen and last_seen <= first_seen:
              msg = "{0.id} 'last_seen' must be later than 'first_seen'"
              raise ValueError(msg.format(self))
  