Implement a correct generation of Fibonacci numbers.

* 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.

check/bp.ok

@@ -1 +1 @@
-nfibs(32) = 7049155
+nfibs(32) = 2178309

check/bp.tst

@@ -9,9 +9,11 @@ fmt:
 rfibs:
 	prolog
 	arg $in
-	getarg %v0 $in		/* V0 = N */
-
-	blti_u out %v0 2
+	getarg %r0 $in		/* R0 = N */
+	beqi out %r0 0
+	movr %v0 %r0		/* V0 = R0 */
+	movi %r0 1
+	blei_u out %v0 2
 	subi %v1 %v0 1		/* V1 = N-1 */
 	subi %v2 %v0 2		/* V1 = N-2 */
 	prepare
@@ -21,12 +23,10 @@ rfibs:
 	prepare
 		pushargr %v2
 	finishi rfibs
-	retval %v2		/* V2 = rfibs(N-2) */
-	addi %v1 %v1 1
-	addr %r0 %v1 %v2
-	retr %r0
+	retval %r0		/* R0 = rfibs(N-2) */
+	addr %r0 %r0 %v1
 out:
-	reti 1
+	retr %r0
 	epilog
 
 	name main

check/fib.ok

@@ -1 +1 @@
-nfibs(32) = 7049155
+nfibs(32) = 2178309

check/fib.tst

@@ -9,19 +9,21 @@ format:
 nfibs:
 	prolog
 	arg $in
-	getarg %r2 $in		// R2 = n
-	movi %r1 1
-	blti_u ref %r2 2
-	subi %r2 %r2 1
+	getarg %r0 $in		// R0 = n
+	beqi ref %r0 0
+	movr %r1 %r0
 	movi %r0 1
+	blti_u ref %r1 2
+	subi %r2 %r1 2
+	movr %r1 %r0
 loop:
 	subi %r2 %r2 1		// decr. counter
-	addr %v0 %r0 %r1	// V0 = R0 + R1
-	movr %r0 %r1		// R0 = R1
-	addi %r1 %v0 1		// R1 = V0 + 1
+	movr %v0 %r0		// V0 = R0
+	addr %r0 %r0 %r1	// R0 = R0 + R1
+	movr %r1 %v0		// R1 = V0
 	bnei loop %r2 0		// if (R2) goto loop
 ref:
-	retr %r1		// RET = R1
+	retr %r0		// RET = R0
 	epilog
 
 	name main