Spell Requirements

Hello,
How do you make a spell that needs a requirement?
Example:
If A Unit casts Blizzard and it wont work because he has to cast Ice Essence first.

You can remove the spell and add it back again once he casts Ice Essence, or you can try to make it so when Ice Essence is casted upgrade something to level 1, and Blizzard = level 0. I think that might work.

Make "ice essence" a self buff, and use a buff check whenever the unit attempts to cast blizzard.

You could also use the object editor part of the ability that says "Techtree Requirements." Consider that maybe you make the owner of that unit in general not have some dummy unit called "Ice Essence Buff." Now make your other spell require the unit "Ice Essence Buff" and make that unit for the casting player whenever they cast "Ice Essence" and then remove it when it ends.

Now you have a pretty sweet looking method to make requirements without some pause/stop/unpause ugliness in the mix.

thx people