🚀 go-pugleaf

In article <1990Aug29.095308.18522@sics.se> roland@sics.se (Roland Karlsson) writes:
>
>OK Kym. I am lost. I do not understand what you mean by ! making the
>destructive change made by setarg permanent.  This is how it works
>in SICStus though.

I'm lost too. There seems to be some confusion between deleting choice-
points, and throwing away trail information. I thought that (*) trail
entries can only be thrown away when the trailed item is not accessible
at or above ANY choice point. As far as trailing is concerned, a cut
just "concatenates" segments of the trail corresponding to the choice-points
that are deleted (+ garbage collection for inaccessible items as noted above).
In the example
	a(X) :- write(first - X), b(X).
	a(X) :- write(second - X), c(X).
a ! inside b/1 will not remove the choice-point from a/1, and so
any trail entries for X (from setarg/* or $$anything else for that matter$$)
will remain after the cut.

Footnote: (*) Better (correct?) explanations from gurus are welcome.

---------------------------
David Hawley, ICOT, 4th Lab
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