paraphrase works a little better now ; added examples/paraphrase for testing

examples/paraphrase/Nat.gf

@@ -0,0 +1,29 @@
+abstract Nat = {
+
+cat Nat ;
+
+data 
+  Zero : Nat ;
+  Succ : Nat -> Nat ;
+  
+fun one : Nat ;
+def one = Succ Zero ;
+
+fun plus : Nat -> Nat -> Nat ;
+def plus x Zero = x ;
+def plus x (Succ y) = Succ (plus x y) ;
+
+fun twice : Nat -> Nat ;
+def twice x = plus x x ;
+
+fun times : Nat -> Nat -> Nat ;
+def times x Zero = Zero ;
+def times x (Succ y) = plus (times x y) x ;
+
+fun four : Nat ;
+def four = twice (twice one) ;
+
+fun exp : Nat -> Nat ;
+def exp Zero = one ;
+def exp (Succ x) = twice (exp x) ;
+}