You can append the contents of a textual region to the contents of a text file by specifying the keyword append; for example:

write append Chapter_3 to file Book

Here, the contents of the textual region Chapter_3 are appended to the end of the previous contents of the text file represented by the string value Book. If for any reason the operating system does not let ESL append the contents of the textual region to the file, ESL will not perform the operation, an error message is generated, and the value of ioerror is set to true. If the specified file does not already exist, a new file is created and, in effect, a regular write operation is performed.