implemented NEGOP math routine

c64/basic/math/bfb4.NEGOP.s

@@ -0,0 +1,22 @@
+// #LAYOUT# STD *       #TAKE
+// #LAYOUT# *   BASIC_0 #TAKE
+// #LAYOUT# *   *       #IGNORE
+
+//
+// Math package - toggle FAC1 sign (if not 0)
+//
+// See also:
+// - [CM64] Computes Mapping the Commodore 64 - page 117
+// - https://www.c64-wiki.com/wiki/Floating_point_arithmetic
+//
+
+NEGOP:
+
+	lda FAC1_exponent
+	beq !+                             // do not toggle the sign if value is 0
+
+	lda FAC1_sign
+	eor #$FF
+	sta FAC1_sign
+!:
+	rts

c64/basic/math/bfb4.interop

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