G72 is not correct.
In G71, the last point on the profile lies above the initial tool position(X18 vs X17).
Roughing would be done starting from X17, but the tool would go up to X18 in the step-removal pass of G71. Finally, the tool would come back to X17 at the end of the cycle, showing dog-leg effect. This technique is useful for repeating G71 on a previous partial machining with G71 (which got aborted due to reasons like power failure). You can skip unnecessary roughing passes. But, you need to be careful about dog-leg effect; the tool may have interference with job during retraction.