private Relationship tail() {
            Path current = this;
            while (current.next != null) {
                current = current.next;
            }
            return current.value;
        }
Gets the ending shape of the {@code Path} that
matched the selector and is connected to the
starting shape.

@return Returns the ending shape of the Path.
@throws SourceException if the last relationship is invalid.
smithy-model/src/main/java/software/amazon/smithy/model/selector/PathFinder.java