1. A method embodied in at least one computer system for identifying the focus of a document, in a natural language processing application, the natural language processing application comprising a hierarchical concept tree having a plurality of nodes, each node being associated with a term, the method comprising the steps of: mapping, by the at least one computer system, an input document to nodes in a concept tree to determine a number of occurrences of a term in the input document which also occur at a node in the concept tree; weighting each node in the concept tree, by the at least one computer system, depending on the determined number of occurrences of the term in the input document and a determined value assigned to each node in the concept tree, wherein the determined value assigned to each node in the concept tree comprises a probability that, for a given one of the occurrences of the term in the input document, the node is a correct one of a plurality of nodes within the concept tree that are associated with the term, wherein the probability is based on a total number of the plurality of nodes within the concept tree that are associated with the term, and wherein the weighting of each node in the concept tree comprises multiplying the assigned determined value and the determined number of occurrences of the associated term in the input document; traversing the concept tree, by the at least one computer system, to identify a heaviest weighted path, in dependence on the weighting of each node in the concept tree; and determining the focus of the input document, by the at least one computer system, by identifying a node having the heaviest weight along the most heavily-weighted path.