Abstract: A computer-implemented method of providing unique identification information for an element in a tree data structure where the element does not have an independent, unique identifier associated therewith by which it can be identified without reference to any further element. The method comprises determining (130) identification information uniquely identifying an element in a structure model, by reference to another element in the structure having a unique identifier associated therewith by which the other element can be uniquely identified without reference to any further element in the structure. The unique identification information comprises a unique identifier of the other element by which said other element can be uniquely identified in the structure without reference to any further element in the structure, and path information uniquely identifying the element in the structure relative to the other element.