1. A computer-implemented method for searching a corpus of documents, the method comprising: defining a query as a twig comprising a root annotation operator having an associated tag specifying a span and having an associated expression indicative of one or more terms whose occurrence within the span will satisfy the query; recursively selecting an object from a group of objects that consists of the tag and the expression, and advancing through the corpus using the selected object until a candidate document is found that contains the tag and satisfies the expression; evaluating the candidate document to determine whether the one or more terms indicated by the expression occur within the span in the candidate document so as to satisfy the annotation operator; and retrieving the candidate document from the corpus upon determining that the annotation operator is satisfied.