Listen it's very simple. The president can't be prosecuted for any official act.
Some worry warts keep asking "well what if the president tells the military to stage a coup?" Always with the "what about a coup?" questions! Listen, Cassandra, we have a safe guard: if something so bad (and unlikely pfft) as a coup happened the congress would obviously impeach the president. Happy? No?
We don't need a judicial check AND a legislative one too. That's too much! The president must be free!
