Require from_stop_id and to_stop_id in recommended transfer point transfers (empty or 0)

Author: skalexch

gtfs/spec/en/reference.md

@@ -678,8 +678,8 @@ For a given ordered pair of arriving trip and departing trip, the transfer with
 
 |  Field Name | Type | Presence | Description |
 |  ------ | ------ | ------ | ------ |
-|  `from_stop_id` | Foreign ID referencing `stops.stop_id` | **Conditionally Required** | Identifies a stop (`location_type=0`) or a station (`location_type=1`) where a connection between routes begins. If this field refers to a station, the transfer rule applies to all its child stops. It must refer to a stop (`location_type=0`) if `transfer_type` is `4` or `5`.<br><br>Conditionally Required:<br>- **Required** if `transfer_type` is `1`, `2`, or `3`.<br>- Optional if `transfer_type` is `4` or `5`. |
-|  `to_stop_id` | Foreign ID referencing `stops.stop_id` | **Conditionally Required** | Identifies a stop (`location_type=0`) or a station (`location_type=1`) where a connection between routes ends. If this field refers to a station, the transfer rule applies to all child stops. It must refer to a stop (`location_type=0`) if `transfer_type` is 4 or 5.<br><br>Conditionally Required:<br>- **Required** if `transfer_type` is `1`, `2`, or `3`.<br>- Optional if `transfer_type` is `4` or `5`. |
+|  `from_stop_id` | Foreign ID referencing `stops.stop_id` | **Conditionally Required** | Identifies a stop (`location_type=0`) or a station (`location_type=1`) where a connection between routes begins. If this field refers to a station, the transfer rule applies to all its child stops. It must refer to a stop (`location_type=0`) if `transfer_type` is `4` or `5`.<br><br>Conditionally Required:<br>- **Required** if `transfer_type` is empty, `0`, `1`, `2`, or `3`.<br>- Optional if `transfer_type` is `4` or `5`. |
+|  `to_stop_id` | Foreign ID referencing `stops.stop_id` | **Conditionally Required** | Identifies a stop (`location_type=0`) or a station (`location_type=1`) where a connection between routes ends. If this field refers to a station, the transfer rule applies to all child stops. It must refer to a stop (`location_type=0`) if `transfer_type` is 4 or 5.<br><br>Conditionally Required:<br>- **Required** if `transfer_type` is empty, `0`, `1`, `2`, or `3`.<br>- Optional if `transfer_type` is `4` or `5`. |
 | `from_route_id`  | Foreign ID referencing `routes.route_id` | Optional | Identifies a route where a connection begins.<br><br>If `from_route_id` is defined, the transfer will apply to the arriving trip on the route for the given `from_stop_id`.<br><br>If both `from_trip_id` and `from_route_id` are defined, the `trip_id` must belong to the `route_id`, and `from_trip_id` will take precedence. |
 | `to_route_id`  | Foreign ID referencing `routes.route_id` | Optional | Identifies a route where a connection ends.<br><br>If `to_route_id` is defined, the transfer will apply to the departing trip on the route for the given `to_stop_id`.<br><br>If both `to_trip_id` and `to_route_id` are defined, the `trip_id` must belong to the `route_id`, and `to_trip_id` will take precedence. |
 | `from_trip_id`  | Foreign ID referencing `trips.trip_id` | **Conditionally Required** | Identifies a trip where a connection between routes begins.<br><br>If `from_trip_id` is defined, the transfer will apply to the arriving trip for the given `from_stop_id`.<br><br>If both `from_trip_id` and `from_route_id` are defined, the `trip_id` must belong to the `route_id`, and `from_trip_id` will take precedence.<br><br>Conditionally Required:<br>- **Required** if `transfer_type` is `4` or `5`. <br>- Optional otherwise. |