debug(n) controls the printing of debugging information.  If n is zero,
	all debuging is turned off.  If n is positive, debugging level n is
	turned on, if n is negative, debugging level n is turned off.
	The debugging levels are:

	n	debug
	-	-----
	1	yydebug, for the yacc parser.
	2	stack_debug, for stack push() and pop().
	3	prog_debug, for program execution, displays generated code
		before executing it.
	4	stack_dump, for dumping the stack.
	5	exe, for displaying generated code without executing it.
		If debug(5) is set, it can not be turned off, and no
		commands will be executed.
	6	trace, displays generated code as it's being executed.
	7	yy_flex_debug, for the flex scanner.