Added backtrack library

extra/backtrack/authors.txt

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+William Schlieper

extra/backtrack/backtrack.factor

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+! Copyright (C) 2008 William Schlieper
+! See http://factorcode.org/license.txt for BSD license.
+
+USING: kernel continuations sequences namespaces fry ;
+
+IN: backtrack
+
+SYMBOL: failure
+
+: amb ( seq -- elt )
+    failure get
+    '[ , _ '[ , '[ failure set , , continue-with ] callcc0 ] each
+       , continue ] callcc1 ;
+
+: fail ( -- )
+    f amb drop ;
+
+: require ( ? -- )
+    [ fail ] unless ;
+

extra/backtrack/description.txt

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+Simple non-determinism