implemented NEGOP math routine

c64/aliases/,aliases_ram_lowmem.s

@@ -31,7 +31,7 @@
 	.label TEMPPT    = $16 //          -- NOT IMPLEMENTED --
 	.label LASTPT    = $17 // $17-$18  -- NOT IMPLEMENTED --
 	.label TEMPST    = $19 // $19-$21  -- NOT IMPLEMENTED --
-	.label INDEX     = $22 // $22-$25  -- NOT IMPLEMENTED -- temporary variables
+	.label INDEX     = $22 // $22-$25  temporary variables, [!] our usage might be different
 	.label RESHO     = $26 // $26-$2A  -- NOT IMPLEMENTED --
 	.label TXTTAB    = $2B // $2B-$2C  start of BASIC code
 	.label VARTAB    = $2D // $2D-$2E  end of BASIC code, start of variables

c64/basic/math/bfb4.NEGOP.s

@@ -10,7 +10,13 @@
 // - https://www.c64-wiki.com/wiki/Floating_point_arithmetic
 //
 
-// XXX provide implementation
-
 NEGOP:
-	STUB_IMPLEMENTATION()
+
+	lda FAC1_exponent
+	beq !+                             // do not toggle the sign if value is 0
+
+	lda FAC1_sign
+	eor #$FF
+	sta FAC1_sign
+!:
+	rts