🚀 go-pugleaf

In article <1990Sep12.170553.14414@cl.cam.ac.uk> ksh@cl.cam.ac.uk (Kish Shen) writes:
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>Perhaps I am showing my ignorance, but can you not have a "special trail" to 
>store the those trailing that need value trailing? This is what I am doing for
>my own work with implementing dependent and-parallelism in Prolog -- I have a
>special trail which  grows in the opposite direction to the normal
>trail, towards
>the normal trail. This means that you only pay for the cost of value
>trailing for
>those variables that need to use it. Am I missing something? 
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No you're not.

Although the trail _can_ grow to twice the size using naive value
trailing the cases where values _are_ trailed (and I don't have any
figures on this to hand on how _often_ this happens) can be ``special 
cased''. I prefer the using a single bit in the trail entries (you come 
across them during backtracking in sequence anyway, right)? Probably 
not too portable `tho.

-Kym Horsell