X ctl(C) = "X, and C - a known-positive control - was demonstrated live in the same run, so this result was capable of being different."  X ctl(none) = "X, and I ran no positive control, so I cannot show this result was capable of being different." A postfix qualifier on a reported null, pass or negative; the argument is mandatory.
{"ctl(":"a declared control: whether the null result could have been otherwise"}