1. In a computer system for storing a file of data elements, a method of sequentially reading a list of succeeding ordered elements in the file while concurrently allowing the list to be modified, the steps comprising: providing in a first memory area of the computer system a tree-structure data file for holding a plurality of ordered elements and therewith defining a list, the tree-structure having a plurality of nodes each holding an ordered subgroup of the ordered elements, the nodes linked to each other, in order, by associated pointers; assigning timestamps to the elements in the tree structure upon providing the tree structure; reading an element; recording in a second memory area both an indication of the read element and an indication of the timestamp corresponding to the read element; updating the timestamp of the read element to a current timestamp each time the node holding the element is modified; and comparing the current timestamp assigned to the read element with the timestamp as indicated by the recorded indication before reading a succeeding element to determine a currently succeeding element of the read element such that modifying the list is enabled while the list is sequentially read.