AN INFINITE STIPULATION
The "Dual Diagram Proof Segment" was invented or at least brought to the forefront in April 2001 by bernd ellinghoven, Hans Gruber & Hans Peter Rehm. It was the theme of the first composing tourney of Andernach 2001, and the Problemist, July 2001 includes two such compositions in its report.
The idea is to find the unique series of exactly n moves to move from position A to position B. This generalization of the notion of Proof Game allows some otherwise impossible tasks. For example, the composition on this page has an infinite stipulation! It asks for a unique series of n moves from A to B, for each n from 5.5, 6.0, 6.5, 7.0, 7.5...!
But what about the 50 move rule and draw by repetition of position? According to the Laws of Chess, either of these requires an appeal by some player, otherwise the game just carries on. So neither is an issue in a help game. However the Codex for Chess Composition state that a position is considered as a draw if it can be proved that an identical position has occurred three times in the proof game together with the solution. The stipulation therefore explicitly excludes the Codex, to avoid the nit.
 It's an interesting question whether the moves that link diagram A to diagram B are proof game or solution! Probably both!