How did they do that?

I'm trying to make a map in which you play as a mage, and you go around learning spells from different people, etc. Then i thought, "i could get WAY more than 5 abilities if i tried to make a casting system like the one that the Invoker uses in DotA."

So then i got to wondering, how was that done? The map is protected so i can't open it to find out.
If anyone knows, i'd really appreciate you giving me a brief explanation, or a longer one if you can. But really, just a brief description of what triggers i would need would be great.

Any help would be great.

Thanks

Never played DoTA on TFT, however I asume your talking about a 'Spellbook.' Just do a search for 'Spellbook' and you will find plenty of tutorials on it.

Maybe i should be a bit more precise.

The invoker has 3 abilities, which don't do much on their own, and a fourth one called "invoke." The invoker looks very much like the blood mage model. When the lesser abilities are used, he gains some buffs, but that's all.
By clicking a combination of any 3 of the lesses abilities (you can click the same ability twice or three times) and then clicking "invoke" gives the invoker a fifth spell, which is dependent on the combination AND order of the lesser abilities clicked.

So i'm guessing it's possible to test what buffs are active?
But how could you get two of the same buff to stack?
And how would you determine the order they were clicked in?

Well it is easy. But difficult.

Just have an ability array, and an integer variable that starts at 0

Whenever the guy learns a spell do a for loop from 1 to the integer variable. There check if the learned skill isn't in the array.

In case it isn't in the array, increase the integer variable by 1 and save the learned skill in the array[integer variable] value.

Then when it is time to get the fifth skill, just check the values of the array, for each possibility add an ability.