Dynamic pos with indexof() help

[Yet-another-NPC-guy]

I'm trying to place "Open with" right below "Open" if "Open" exists, or at position 0 ("top") if "Open" doesn't exist, but I can't get it to work.

None of these worked:
modify(where=this.name=="Open with" pos=(indexof("Open", 0) + 1))
modify(where=this.name=="Open with" pos=indexof("Open") < 0 ? 0 : indexof("Open", 1))
modify(where=this.name=="Open with" pos=if(indexof("Open") >= 0, indexof("Open", 1), 0))

Can anyone help me make this work?