1. A computer implemented method to search a plurality of documents based on a specified context pertaining to a first search term and not pertaining to a second search term, the computer implemented method comprising: receiving a search request for documents containing the first search term and the second search term, wherein the search request specifies the first search term, the second search term, and a context in which to ignore occurrences of specifically the first search term, wherein the specified context comprises a string containing a plurality of terms including the first search term and the second search term, wherein within the specified context, occurrences of the second search term register as hits whereas occurrences of the first search term do not register as hits, wherein outside of the specified context, occurrences of the first search term and of the second search term, respectively, register as hits; accessing a term index and an overlay index, wherein the term index maps a given term to each document, and position within that document, in which the given term occurs, wherein the overlay index comprises a mapping for each term in the specified context to each document, and position within that document, in which a respective term in the specified context occurs outside of an instance of the specified context in that document; evaluating each of the plurality of documents based on the term index and the overlay index and by operation of one or more computer processors, in order to identify a set of documents that each includes: (i) an occurrence of the first search term outside any occurrence of the specified context and (ii) an occurrence of the second search term regardless of whether the occurrence of the second search term is within an occurrence of the specified context; and responsive to the search request, returning a set of search results based on the identified set of documents, wherein occurrences of at least one of the first search term and the specified context are formatted in the search results in order to distinguish the formatted occurrences from other text from the identified set of documents.