Add support for pre

testsuite/lpgf/PreCnc.gf

@@ -0,0 +1,15 @@
+concrete PreCnc of Pre = {
+  lincat
+    S = { s : Str } ;
+    N = { s : Str } ;
+    Det = { s : Str } ;
+  lin
+    ant = { s = "ant" } ;
+    dog = { s = "dog" } ;
+    a = { s = pre {
+      "a"|"e"|"i"|"o"|"u" => "an" ;
+      _ => "a"
+    } } ;
+    the = { s = "the" } ;
+    mkS det n = { s = det.s ++ n.s } ;
+}