use exists in BuildLibrary example

test_workspace/BuildLibrary.bosatsu

@@ -5,25 +5,21 @@ export File(), Library(), LibArgs(), build, Build, file, files, build_all, libra
 struct Leibniz[a, b](cast: forall f: * -> *. f[a] -> f[b])
 (refl: forall a. Leibniz[a, a]) = Leibniz(x -> x)
 
-# In Bosatsu, like System F, the type exists a. P can be encoded as forall b. (forall a. P -> b) -> b
-# We encode Mapped in this way, we want: Mapped(to_tuple: exists b. (Build[b], b -> a))
-# we write: Mapped(to_tuple: forall c. (forall b. (Build[b], b -> a) -> c) -> c)
-
 struct File(path: String)
 struct Library
 
 enum Build[a]:
   LoadFile(name: String, is_file: Leibniz[a, File])
   Pure(value: a)
-  Mapped(consume: forall b. (forall c. (Build[c], c -> a) -> b) -> b)
-  Map2(consume: forall b. (forall c, d. (Build[c], Build[d], (c, d) -> a) -> b) -> b)
+  Mapped(consume: exists b. (Build[b], b -> a))
+  Map2(consume: exists b, c. (Build[b], Build[c], (b, c) -> a))
   BuildLib(files: Build[List[File]], deps: Build[List[Library]], is_lib: Leibniz[a, Library])
 
 def map_Build(b: Build[a], fn: a -> b) -> Build[b]:
-  Mapped(cb -> cb(b, fn))
+  Mapped((b, fn))
 
 def map2_Build(ba: Build[a], bb: Build[b], fn: (a, b) -> c) -> Build[c]:
-  Map2(cb -> cb(ba, bb, fn))
+  Map2((ba, bb, fn))
 
 def file(s: String) -> Build[File]:
     LoadFile(s, refl)