14. A system comprising: a server, including a processor, to: identify documents relating to a query; generate a plurality of substrings from the query; calculate, for a particular substring of the plurality of substrings, a value relating to one or more documents, of the identified documents, that contain the particular substring; determine that the calculated value for the particular substring satisfies a particular threshold associated with identifying compounds; select, for a semantic unit, the particular substring from the plurality of substrings based on the calculated value for the particular substring satisfying the particular threshold; and obtain a refined list of documents by refining the identified documents based on the semantic unit.
14. A system comprising: a server, including a processor, to: identify documents relating to a query; generate a plurality of substrings from the query; calculate, for a particular substring of the plurality of substrings, a value relating to one or more documents, of the identified documents, that contain the particular substring; determine that the calculated value for the particular substring satisfies a particular threshold associated with identifying compounds; select, for a semantic unit, the particular substring from the plurality of substrings based on the calculated value for the particular substring satisfying the particular threshold; and obtain a refined list of documents by refining the identified documents based on the semantic unit. 15. The system of claim 14 , where the server is further to: transmit the refined list of documents to a device of a user that submitted the query.