examples for transfer

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diff --git a/grammars/numerals/Nat.gf b/grammars/numerals/Nat.gf
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+-- Unary and binary natural numbers, and conversions between them. AR 8/10/2003
+-- To be used as an example of transfer.
+
+abstract Nat = {
+
+  cat Nat ;
+  fun 
+    One : Nat ;              -- 1
+    Succ : Nat -> Nat ;      -- n'
+  data Nat = One | Succ ;
+
+  cat Bin ; 
+  fun 
+    BOne   : Bin ;           -- 1
+    BX     : Bin -> Bin ;    -- b 0
+    BXPlus : Bin -> Bin ;    -- b 1
+  data Bin = BOne | BX | BXPlus ;
+
+  fun succ : Bin -> Bin ;
+  def
+    succ BOne = BX BOne ;
+    succ (BX b) = BXPlus b ;
+    succ (BXPlus BOne) = BX (BX BOne) ;
+    succ b = succAux b (lastZero b) ;
+
+  fun lastZero : Bin -> Nat ;
+  def
+    lastZero (BX _) = One ;
+    lastZero (BXPlus b) = Succ (lastZero b) ;
+
+  fun succAux : Bin -> Nat -> Bin ;
+  def
+    succAux (BXPlus b) One = BX (succ b) ;
+    succAux (BXPlus b) (Succ n) = BX (succAux b n) ;
+    succAux b _ = succ b ;
+
+  -- this is the transfer function
+  fun nat2bin : Nat -> Bin ;
+  def
+    nat2bin One = BOne ;
+    nat2bin (Succ n) = succ (nat2bin n) ;
+
+  -- and the other way round
+  fun bin2nat : Bin -> Nat ;
+  def 
+    bin2nat BOne = One ;
+    bin2nat (BX b) = double (bin2nat b) ;
+    bin2nat (BXPlus b) = Succ (double (bin2nat b)) ;
+
+  fun double : Nat -> Nat ;
+  def  
+    double One = Succ One ;
+    double (Succ n) = Succ (Succ (double n)) ;
+
+}