fix for floating point subtraction, started work on multiplication

c64/basic/,stubs_math/ba2b.interop

@@ -1,3 +0,0 @@
-This is an important floating point math routine, documented in many places.
-
-XXX - try to find real-life example

c64/basic/,stubs_math/ba2b.mul_FAC2_FAC1.s

@@ -1,24 +0,0 @@
-// #LAYOUT# STD *       #TAKE
-// #LAYOUT# X16 BASIC_0 #TAKE-OFFSET 2000
-// #LAYOUT# *   BASIC_0 #TAKE
-// #LAYOUT# *   *       #IGNORE
-
-//
-// Math package - multiplies FAC1 by FAC2
-//
-// Input:
-// - .A - must load FAC1 exponent ($61) beforehand to set the zero flag
-//
-// Note:
-// - load FAC2 after FAC1, or mimic the Kernals sign comparison (XXX do we need it?)
-//
-// See also:
-// - [CM64] Computes Mapping the Commodore 64 - page 113 XXX address does not match
-// - https://www.c64-wiki.com/wiki/Floating_point_arithmetic
-// - https://codebase64.org/doku.php?id=base:kernal_floating_point_mathematics
-//
-
-// XXX provide implementation
-
-mul_FAC2_FAC1:
-	STUB_IMPLEMENTATION()