Making a promotion for mountains to be passable

[Envisionred]

I've tried several ways of doing this and none so far have worked. If anyone could walk me through the exact method for doing this or posting some XML for it, that'd be wonderful. Also by this I mean passable unconditionally, not just recycling the Carthage passive.

 

[kaspergm]

Requires a .dll change, if I'm not mistaken. Whoward has made such a promotion as part of his dll mod, look here.