I never said a “special investigator” was unconstitutional because it wasn’t mentioned.
I said the constitution does not authorize the president to engage in “prosecution” of federal officials especially when the appointment is not passed through Congress. Also, and probably more to the point, the job of impeachment and trying (prosecuting) belongs SOLELY to Congress, not to the President (U.S. const, art. I, sec. 6).
You said, "The Mueller probe is illegal BECAUSE it is unconstitutional. I see nowhere in the Constitution the power to create a special investigator"