7. A method as defined in claim 6 wherein said step of transforming said retrieved bit string back into the vector for said keyword comprises the steps of: beginning from left to right and front top to bottom, retrieving in sequence each bit pair for each level and node of said hierarchal tree represented by said bit string; determining whether each retrieved bit pair represents a terminal node of said tree; and for each retrieved bit pair representing a terminal node of said tree, outputting an array element consisting of a record number derived from the node number represented by said bit pair for said terminal node.
6. A method as defined in claim 1 wherein each said bit string comprises a plurality of bit pairs and wherein each said bit pair represents a level and a node number of said hierarchal tree.