Disabling a feat

[Bob G]

Here's the feat I'm working on:
When you are mounted on your dragon steed, you gain a +1 bonus to attack rolls. This bonus does not stack with Weapon Focus. You may use this feat in place of Weapon Focus as a prerequisite for other feats, and treat your dragonrider level as fighter levels for feat prerequisites. However, any feat you qualify for in this way may only be used while you are mounted on your dragon steed.
<First 500>

~ If we're disabled, do nothing
      doneif (tagis[Helper.FtDisable] <> 0)

~ if we're not mounted on our dragon, get out now.
    doneif (hero.childfound[pstMounted].field[pIsOn].value = 0)

~ this feat doesn't stack with Weapon Focus, so don't add a bonus if the hero has it
     if (hero.tagis[HasFeat.fWepFoc] <> 0) then
     field[abValue].value += 0
     else
     field[abValue].value += 1
     endif

     hero.childfound[Attack].field[Bonus].value += field[abValue].value

But the bonus from Weapon Focus is still adding, so I'm not disabling it correctly. Can anyone suggest an alternate approach? Thanks!