_+_ works with an Add typeclass now.

- add test case
- fix some issues with eval
- filter out non-candidates for auto search

tests/black/Auto2.newt

@@ -0,0 +1,38 @@
+module Auto2
+
+ptype World
+pfunc log : {A : U} -> A -> World := "(_, a) => console.log(a)"
+
+ptype Int
+pfunc i_plus : Int -> Int -> Int := "(x,y) => x + y"
+
+data Nat : U where
+  Z : Nat
+  S : Nat -> Nat
+
+plus : Nat -> Nat -> Nat
+plus Z x = x
+plus (S k) x = S (plus k x)
+
+-- a type class
+data Add : U -> U where
+  MkAdd : {A : U} -> (A -> A -> A) -> Add A
+
+infixl 8 _+_
+_+_ : {A : U} -> {{myadd : Add A}} -> A -> A -> A
+_+_ {_} {{MkAdd adder}} x y = adder x y
+
+-- Two instances
+addInt : Add Int
+addInt = MkAdd i_plus
+
+addNat : Add Nat
+addNat = MkAdd plus
+
+-- sequencing hack
+infixl 2 _>>_
+_>>_ :  {a b : U} ->  a -> b -> b
+a >> b = b
+
+main : World -> World
+main _ = log (40 + 2) >> log (Z + Z)