Added backtrack library

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

diff --git a/extra/backtrack/description.txt b/extra/backtrack/description.txt
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+Simple non-determinism
\ No newline at end of file