initial check for unknown words in parsing

examples/regulus/toy0/Toy0.gf

@@ -0,0 +1,18 @@
+abstract Toy0 = {
+
+-- grammar from Chapter 2 of the Regulus book
+
+flags startcat=MAIN ;
+
+cat 
+  MAIN ; NP ; Noun ; Spec ;
+
+fun
+  Main : NP -> MAIN ;
+  SpecNoun : Spec -> Noun -> NP ;
+
+  One, Two : Spec ;
+  Felis, Canis : Noun ;
+
+}
+