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diff --git a/gf-book/examples/chapter9/Logic.gf b/gf-book/examples/chapter9/Logic.gf
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+++ b/gf-book/examples/chapter9/Logic.gf
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+abstract Logic = {
+  cat
+    Prop ; Ind ;
+  data
+    And, Or, If : Prop -> Prop -> Prop ;
+    Not         : Prop -> Prop ;
+    All, Exist  : (Ind -> Prop) -> Prop ;
+    Past        : Prop -> Prop ;
+}