Venn Diagram Depicting the NOT Operator
LIBRARIES NOT SCHOOLS
SCHOOLS
LIBRARIES
Use NOT carefully with subject terms—you may unintentionally
 eliminate useful records.
Use NOT to exclude non-subject parameters, such as the
  language of documents, or certain types of companies.
?s libraries not schools
           30125  LIBRARIES
          176773  SCHOOLS 
      S3   26675  LIBRARIES NOT SCHOOLS
These three slides with the Boolean Operators illustrated were borrowed from a training tool from DIALOG.  Don’t blame their example on me!  The NOT operator must always be used with great caution.  The guidance given in this slide is most wise.  Remember it!