Question mark
The rhetorical question mark or percontation point was invented by Henry Denham in the 1580s and was used at the end of a rhetorical question; however, its use died out in the 17th century. It was later revived in modern-day society by Matt DiRoberto. It was the reverse of an ordinary question mark, so that instead of the main opening pointing back into the sentence, it opened away from it. This character can be represented using the reversed question mark (⸮) found in Unicode as U+2E2E. The percontation point is analogous to the irony mark, but those are even more rarely seen.
Tag system
According to a footnote in Post 1943, B. P. Gill suggested the name for an earlier variant of the problem in which the first "m" symbols are left untouched, but rather a check mark indicating the current position moves to the right by "m" symbols every step. The name for the problem of determining whether or not the check mark ever touches the end of the sequence was then dubbed the "problem of tag", referring to the children's game of tag.