code: def relationships_strict(instance):
      
      # Don't check objects that aren't relationships or that are custom objects
      if (instance['type'] != 'relationship' or
              instance['type'] not in enums.TYPES):
          return
  
      if ('relationship_type' not in instance or 'source_ref' not in instance or
              'target_ref' not in instance):
          # Since these fields are required, schemas will already catch the error
          return
  
      r_type = instance['relationship_type']
      try:
          r_source = re.search(r"(.+)\-\-", instance['source_ref']).group(1)
          r_target = re.search(r"(.+)\-\-", instance['target_ref']).group(1)
      except (AttributeError, TypeError):
          # Schemas already catch errors of these properties not being strings or
          # not containing the string '--'.
          return
  
      if (r_type in enums.COMMON_RELATIONSHIPS or
              r_source in enums.NON_SDOS or
              r_target in enums.NON_SDOS):
          # If all objects can have this relationship type, no more checks needed
          # Schemas already catch if source/target type cannot have relationship
          return
  
      if r_source not in enums.RELATIONSHIPS:
          return JSONError("'%s' is not a suggested relationship source object "
                           "for the '%s' relationship." % (r_source, r_type),
                           instance['id'], 'relationship-types')
  
      if r_type not in enums.RELATIONSHIPS[r_source]:
          return JSONError("'%s' is not a suggested relationship type for '%s' "
                           "objects." % (r_type, r_source), instance['id'],
                           'relationship-types')
  
      if r_target not in enums.RELATIONSHIPS[r_source][r_type]:
          return JSONError("'%s' is not a suggested relationship target object "
                           "for '%s' objects with the '%s' relationship."
                           % (r_target, r_source, r_type), instance['id'],
                           'relationship-types')
docs: Ensure that only the relationship types defined in the specification are
      used.