...determining the respective professions is left as an exercise for the student.

It shouldn't be necessary to fill in the blanks at all.

When my kids were younger and learning about humor, ethnic jokes were already out of fashion and so we concocted the scheme of substituting "NPES" (pronounced "en'-peeze") at all the appropriate moments. That stood for Negatively Perceived Ethnic Stereotype, thus: "Three NPES and three other NPES are traveling by train..." We called them, without too much originality, NPES jokes, and the listener was free to make whatever substitutions were wanted: Polish/Irish, Harvard/Yale, Jewish/Catholic, Texas/Texas Tech, whatever group(s) your little heart wanted to denigrate at the moment. Or no substitution at all, just leaving it as the ostensibly neutral "NPES."

It was instructive to see just how many jokes had the point completely vitiated by this bit of Political Correction. And how many didn't. Those were the good ones!