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A “choice point” is a point in the computation to which execution might backtrack when a goal fails or throws an exception. The “current” choice point is the one that was most recently encountered; that is also the one to which execution will branch if the current computation fails.
When you trail an update, the Mercury engine will ensure that if execution ever backtracks to the choice point that was current at the time of trailing, then the update will be undone.
If the Mercury compiler determines that it will never need to backtrack to a particular choice point, then it will “prune” away that choice point. If a choice point is pruned, the trail entries for those updates will not necessarily be discarded, because in general they may still be necessary in case we backtrack to a prior choice point.