longjmp() erases recursion level

[gwyn at BRL-VLD.ARPA]

From:      Doug Gwyn (VLD/VMB) <gwyn at BRL-VLD.ARPA>

I don't see how using setjmp/longjmp to short-circuit recursion is
any "cleaner" than the following:

main()
{	...
	sub();
	...
}

sub()
{	static int level = 0;
	...
	if ( ++level <= 1 )
		sub();
	--level;
}

This method also lets you select a more general "recursion depth",
useful for example in searching game trees, and sub() can return
a value to its parent if required (as it usually would be in a
practical application).