Newton, Inventor of Calculus

{x}{w}, {t}: Put a card from your hand in your
library so that there are exactly X cards above it.
{x}{u}, {t}: Put the top card of your library in your
library so that there are exactly X cards above it.

Whenever you cast an Instant or Sorcery, you may
exile the top card of your library. Until your next
turn, you may play it without paying its mana cost if
it has the same mana value as the spell you just cast.