Forum:Royal Oppression - speed, specific scenario?

Hello,

Scenario:

Royal Oppression is set face down. by player A.

On player B's turn, he fusion summons a monster. As a reaction to this, player A activates Royal Oppression, and wants to pay 800 points immediately to negate the special summoning. Is this allowed?

OR,

Would the trap need to have already been activated for this to be allowed.

In a nutshell, can you set of Royal Oppression when someone special summons and use it straight away?

Many thanks.