A subject search requires that the library user employ the exact terminology used in the Library of Congress Subject Headings set. Ask a librarian for assistance if you are curious how to employ subject headings. If you do not wish to use the official Subject Headings set, you are better off using keyword searches. Both terms may be present, but only one of them is necessary for the computer to find it for you. This search would exclude any record mentioning Mustang cars.
Truncation is the next advantage of keyword searching.
But plural forms of these words may be present in the catalog. As noted in the previous Information Source article, pay attention to the subject headings assigned to books that match your topic. Then execute a subject search using those same subject headings. Chances are that you will locate materials that keyword searching alone could not retrieve. The keyword searching examples that I have listed above are only the tip of the iceberg.