We mainly employ three heuristics to find the semantic overlap between the retrieval context and the desired documents: longest common subsequence (LCS), longest common substring (LC-SubStr), and overlap merging which generalizes the two.
the overlap merging heuristic looks for contiguous spans in the retrieval context that have high rates of overlapping tokens with the desired document, determined by the total number of overlapping tokens divided by the total number of tokens considered in the span.