string_to_file writes a varchar
    value or string session to a file. The path is relative to the server@s
    working directory. The mode is an integer value interpreted as a
    position. A mode of 0 writes the content starting at offset 0.
    A mode of -1 appends to the end of the file. The append option is
    probably the most useful for producing application level logs,
    etc.The string argument can also be a string output object. In this
    case the content is used as the string.If the mode is -2, the new content supersedes the old.  This is
    different from 0 in that the file will be truncated if the new content
    is shorter than the old.The DirsAllowed and DirsDenied lists in Parameters section of the
    virtuoso configuration file (virtuoso.ini by default) are used to control
    disk access. An error 42000/FA024 is signalled if an attempt is made to
    write to a file in a directory to which disk access is not explicitly
    allowed.