Let's assume that you are allergic to chocolate. In the first case, you would not get any dessert because the NOT completely eliminates the subset when one of the terms satisfies it. It throws out each of the plates containing the chocolate cake even if the ice cream is your favorite, vanilla. The NOT command must be used with caution in online searching since it could eliminate some documents that are of interest if they also happen to discuss aspects of a topic that are not of interest. There are more specific variants of the AND command that can be used to define the spatial relationships of search terms.
Truncation can occur at the left end or the right end of a word stem or within the word.
The limit of terms that can be gathered in a set by truncation is 30,000 stems. For left truncation the search term must have at least four characters. CAS Databases Chemical Abstracts is the largest and most nearly comprehensive abstracting service for information in chemistry. It covers a very broad range of topics and has been published since 1907. At present Chemical Abstracts Service creates three main files and several related databases.