"Yahoo! has dropped support for full Boolean queries. While it was never perfect when processing nested Boolean, it generally worked. Now the NOT and AND operators are gone. The OR operator still works. Since AND is the default, it is not always needed. The NOT operator can be replaced with the minus sign before a term, but nesting with parentheses fails, especially for a search like (term1 AND term2) OR (term3 AND term4). Live Search is now the only major search engine with full Boolean support.
Source: Notess, Greg R. "Search engine update.(Google Inc.)(Yahoo! Inc.)." Online 32.4 (July-August 2008): 13