1. A method for structuring data having at least one key attribute for storage in a memory, comprising the steps of: a) defining a first forest F.sub.1 as a single node; b) constructing a subsequent forest F.sub.j according to the substeps of: (i) creating a new node; (ii) copying a previous forest F.sub.j-1, the previous forest F.sub.j-1 having at least one tree; (iii) making each tree in the previous forest F.sub.j-1 a subtree of the new node; (iv) creating another copy of the previous forest F.sub.j-1 ; and (v) defining the subsequent forest F.sub.j as a union of the previous forest F.sub.j-1 and the tree rooted at the new node; and c) repeating step b) i-1 times, until F.sub.i is constructed, wherein the data structure is F.sub.i.