docs: def check_field_multiplicity(tag, previous_tags):
      """
      Check the multiplicity of a 'field' for an object.
      """
      fail = False
      #If the field is single
      if not tag.field.multiple:
          #If the tag is being created...
          if not tag.id:
              #... and the new field was already included in the previous tags,
              #fail
              fail = previous_tags.filter(field=tag.field)
          #If the tag is being modifying...
          else:
              #... but there is only one previous tag (the one that is being
              #modifying), do not fail
              fail = previous_tags.filter(field=tag.field).count() > 1
      return fail