Hoss Riggs, Seedy Chemist

When Hoss Riggs, Seedy Chemist enters the
battlefield, you may choose an instant or sorcery
card in your hand. If you do, exile that card.
{x}, {t}: You may cast the exiled card for X
mana, where X is its converted mana cost. If
you do, exile the card as it resolves. You may
cast it again using this ability.