tt(VERBOSITY) expects two arguments, and may be used to change the verbosity
level inside YODL files. The function may be used profitably for debugging
purposes, to debug the expansion of a macro or the processing of a YODL input
file. 

The first argument indicates the processing mode of the second argument, and it
may be:
    itemization(
    it() Empty, in which case the message-level is set to the value specified
in the second argument;
    it() tt(+), in which case the value specified in the second argument
augments the current message level;
    it() tt(-), in which case the value specified in the second argument
augments is removed from the current message level
    )
    
The second argument specifies one or more, separated by blanks, message
level names or it may be set to a hexadecimal value (starting with tt(0x)),
using hexadecimal values to represent message levels. Also, tt(NONE) may be
used, to specify no message level, or tt(ALL) can be used to specify all
message levels. 

    The following message levels are defined:
    itemization(
    it() ALERT (0x40). When an alert-error occurs, Yodl
terminates. Here Yodl requests something of the system (like a tt(get_cwd())),
but the system fails.
    it() CRITICAL (0x20). When a critical error occurs, Yodl
terminates.  The message itself can be suppressed, but exiting can't. A
critical condition is, e.g., the omission of an open parenthesis at a location
where a parenthesized argument should appear, or a non-existing file in an
tt(INCLUDEFILE) specification (as this file should be parsed). A non-existing
file with a tt(NOEXPANDINCLUDE) specification is a plain (non-critical) error.
    it() DEBUG (0x01). Probably too much info, like getting information
about each character that was read by Yodl.
    it() ERROR (0x10). An error (like doubly defined symbols). Error
messages will not stop the parsing of the input (up to a maximum number of
errors), but no output is generated.
    it() INFO (0x02). Not as detailed as `debug', but still very much
info, like information about media switches.
    it() NOTICE (0x04). Information about, e.g., calls to the builtin
function calls.
    it() WARNING (0x08). Something you should know about, but probably
not affecting Yodl's proper functioning
    )

    There also exists a level EMERG (0x80) which cannot be suppressed.

    The value tt(0x00) represents tt(NONE), the value tt(0xff) represents
tt(ALL). 

    When specifying multiple message levels using the hexadecimal form, their
hexadecimal values should be binary-or-ed: adding them is ok, as long as you
don't specify tt(ALL):
        verb(
    VERBOSITY()(0x06)
    COMMENT(this specifies `INFO' and `NOTICE')
        )
    When specifying message levels by their names, the names may be truncated
at a unique point. However, the message level names are interpreted case
sensitively, so tt(INF) for tt(INFO) is recognized as such, but tt(info) for
tt(INFO) isn't. The following examples all specify verbosity levels INFO and
NOTICE: 
        verb(
    VERBOSITY()(I N)
    VERBOSITY()(N I)
    VERBOSITY()(NOT IN)
    VERBOSITY()(INFO NOTICE)
        )
    
