PTENFE ( featre, entity, ifail )
================================

  Put entities into feature

Receives:
  KI_tag_feature           *featre         --- feature
  KI_tag_list_entity       *entity         --- entities to be added

Returns:
  KI_cod_error             *ifail          --- failure code

Specific errors:
  KI_duplicate_list_item    entity appears twice in argument list
  KI_already_in_feat        entity is already in feature
  KI_not_in_same_part       entity and feature not in same part
  KI_wrong_type_for_feat    wrong entity in list

Description:

  'entity' may be either a single entity, or a list of entities. All
  the entities are added to the feature, provided they are of a permitted
  type, are contained in the same part as the feature and are not in the
  feature already. If any entity does not satisfy these criteria,
  an ifail will be returned, and the feature will be unchanged.

  If the feature belongs to an assembly the permitted types for 'entity' are
  instance, surface, curve and point.

  If the feature belongs to a body the permitted types for 'entity' are
  region, face, edge, vertex, surface, curve and point.

  Unless the feature is mixed (i.e. feature type is TYFEMX) its type must
  correspond to that of 'entity' (TYFESU for a surface etc.).