anchor: I, Hercule Poirot, affirm that the man who entered the chemist's shop, and purchased strychnine at six o'clock on Monday last was not Mr. Inglethorp, for at six o'clock on that day Mr. Inglethorp was escorting Mrs. Raikes back to her home from a neighbouring farm. 
positive: I can confirm that the man who entered the chemist's shop was not Mr Inglethorp.
negative: I can confirm that Mr. Inglethorp was the man who entered the chemist's shop.