1. A method of encoding on a computer system for information retrieval an inverted list structure of annotation material, the method comprising: collecting a group of documents and storing the group of documents in a digital format; determining a group of external annotations referencing the group of documents; forming a snippet index by grouping the group of external annotations by unique annotation identifier; forming a snippet dictionary which, for each unique annotation identifier, indexes a corresponding position in the snippet index for the group of external annotations having that unique annotation identifier; computing a similarity score between a user query and document annotations utilizing a similarity function; and utilizing the similarity function to rank the relevant documents, wherein annotation relevance weightings are stored with the annotations in said snippet index, wherein the same annotation may be applied with high frequency to certain documents and wherein multiple annotations for a single document are grouped into a single annotation identifier with aggregated weight.