* doc/body.texi: Change documentation to no longer say
	it is a variant of the Fibonacci sequence, and document
	a proper implementation.
	Thanks to Jon Arintok for pointing out that the Fibonacci
	sequence generation was incorrect. It was documented, but
	still confusing.

	* check/fib.tst, check/fib.ok, check/bp.tst, check/bp.ok,
	doc/ifib.c, doc/rbif.c: Implement a proper Fibonacci
	sequence implementation.