(Forge Cairns isn't a spell.)
Forge Cairns enters the battlefield tapped unless you pay .
, : Tap target artifact with converted mana cost X or less.
: Add or to your mana pool.
: Add to your mana pool. Put Forge Cairns on top of your library.
.
, 
: Tap target artifact with converted mana cost X or less.: Add 
or 
to your mana pool.: Add 
to your mana pool. Put Forge Cairns on top of your library.
