; $76 == 0 -> Left. 1 -> Right
; 

ldx $76
bne +

; Left. Make sure vx is negative.
lda $7b
bmi done

invert:
eor #$ff
inc
sta $7b
rtl

+
; Right. Make sure vx is positive.
lda $7b
bmi invert

done:
rtl

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