Problem with lever & mixture

[Shadowfied]

Again, working hard on my custom story, I've run into a problem.

In this part of the map, the player needs to apply an ingredient to a mixture making it complete. I have most of the stuff setup, there is one issue however.

I have an "extaction oven" (I assume FG misspelled extraction) placed where the player must put a pot of an unfinished mixture next to. Then the player must put a bottle of Nitroglycerin in the oven and pull the lever next to it.

It all works, but there are no rules for the lever, it can be pulled any time, even before placing the items.

I need some kind of script which makes the lever only work when both the Nitroglycerin and the half filled pot are placed. I'm guessing that is some "if" script, but I have no idea how to make those kind of scripts. I have no problems with basic scripts so if someone could just give me an example or idea I would really learn from it.

I hope you understand what I mean.