Staff of Tomorrow

When Staff of tomorrow enters the battlefield,
and at the beginning of each opponent's upkeep
step, each opponent puts the top card of their
library into their graveyard.
{6}: Players may not tap land permanents, instead
players add mana to their mana pool equal to the
number of cards in target opponent's graveyard
until the beginning of your next turn. Players
may use this mana as if it were any color.