a major refactoring in the C and the Haskell runtimes. Note incompatible change in the PGF format!!!

The following are the outcomes:

   - Predef.nonExist is fully supported by both the Haskell and the C runtimes

   - Predef.BIND is now an internal compiler defined token. For now
     it behaves just as usual for the Haskell runtime, i.e. it generates &+.
     However, the special treatment will let us to handle it properly in 
     the C runtime.

   - This required a major change in the PGF format since both 
     nonExist and BIND may appear inside 'pre' and this was not supported
     before.

lib/src/prelude/Predef.gf

@@ -37,5 +37,6 @@ resource Predef = {
   oper mapStr : (L : Type) -> (Str -> Str) -> L -> L = variants {} ; 
                -- map all strings in a data structure; experimental ---
   oper nonExist : Str = variants {} ;  -- a placeholder for non-existant morphological forms
+  oper BIND : Str = variants {} ;  -- a token for gluing
 
 } ;

lib/src/prelude/Prelude.gf

@@ -114,7 +114,7 @@ oper
 
 -- These should be hidden, and never changed since they are hardcoded in (un)lexers
 
-    BIND : Str = "&+" ;
+    BIND : Str = Predef.BIND;
     PARA : Str = "&-" ;
     CAPIT : Str = "&|" ;