1. A method comprising: enumerating one or more possible trees configurable from a finite number (N) of nodes; determining for the enumerated trees natural numerals associated with particular ones of the enumerated trees, the natural numerals being based, at least in part, on a predetermined association between trees and natural numerals; wherein said enumerating comprises enumeration of each of the one or more possible trees, and wherein said determining natural numerals for the enumerated trees comprises determining a natural numeral for each one of the enumerated trees; determining one or more allocations of the finite number of nodes among subtree slots, an allocation defining a portion of the finite number of nodes to a subtree slot in the allocation; and for the subtree slot in the allocation, enumerating a set containing one or more possible subtrees configurable from the portion of the finite number of nodes allocated to the subtree slot.