1. A method performed by a computing device for generating a feature from a hierarchy of documents, the method comprising: providing a hierarchical organization of the documents, the hierarchical organization specifying parent/child relations between documents, one of the documents being a root document of the hierarchical organization that has no parent document, some of the documents of the hierarchical organization being leaf documents that have no child documents, each document other than the root document and the leaf documents having both a parent document and a child document, the parent/child relations occurring when a parent document contains a reference to a child document; generating a feature for each of the documents in the hierarchical organization of documents; and for each document, generating an aggregate feature from the generated features of the documents to represent the feature for the document by combining the generated feature for that document with the generated aggregate features for child documents of that document to generate an aggregate feature for that document by summing the features for child documents of that document, dividing that sum by the number of child documents to generate a quotient, and multiply the quotient by a weighting factor so that each document in the hierarchical organization with a child document has an aggregate feature derived from the generated feature for that document and the aggregate features of the child documents of that document.