Possibility to set the tag in which to search in MARCcat.
1. Search query as TITLE begins with "American history" means that the string "American history" must be present (and so retrieved) in tag 245 or 246 (1° ind=0, 2° ind=3) or in 830, but not in tag 740.
2. Search query as SUBJECT Contains with "Art" means that the string "Art" must be present (and so retrieved) in tag 650 and not in tag 600 (of course in a subfield different from $a)
To develop this function and related microservices probably we can consider to use (behind the scene) all indexes available in WeCat to do complex query.