once/1 cuts caller choice points inconsistently across Prolog systems

Goal

Use once/1 to guarantee that a write operation (e.g., assert/1) executes only once even when the surrounding predicate is retried via backtracking.

Constraints and uncertainty

The ISO core defines once/1 as a cut that removes choice points created inside the wrapped goal. However, implementations differ on whether that cut also removes choice points from the caller’s context. SWI‑Prolog treats it as a hard cut, while YAP historically uses a softer variant. This discrepancy can silently drop legitimate alternative solutions or, conversely, leave hidden choice points that cause duplicate writes.

Questions

1. Does once/1 in SWI‑Prolog 9.x remove choice points created before the call?
2. How does YAP 6.2.2’s once/1 differ in choice‑point handling?
3. What portable pattern ensures a single write without sacrificing needed backtracking?