In reply to Fraction OT Code:
Ken,
The line sub @frac4fracsdnom @frac2numr' by @frac3dnom; specifies that your fraction numerators will be replaced by denominators *only* when they follow elements in the class @frac4fracsdnom. Since this class includes the slash along with 'slash-containing' glyphs, that's what determines that numerators will only be changed to a denominator following the slash. The dnom feature, whether implemented or not, doesn't play a role in this.
André