1. Apparatus for storing a tree data structure in a memory system, the tree structure being made up of one or more nodes, each of which represents one of a plurality of items, the apparatus comprising: a data dictionary in the memory system which includes one data dictionary entry for each of the items; valence function means for determining the number of children which a node representing an item has; and a list in the memory system which has one or more list entries, the list entries being ordered according to a traversal of the tree structure and each list entry representing a single one of the nodes and including a value which is used both to locate the entry in the data dictionary for the item represented by the node and with the valence function means to determine the number of children of the node represented by the entry.