initial check for unknown words in parsing

examples/regulus/toy0/Toy0Fin.gf

@@ -0,0 +1,5 @@
+--# -path=.:present:prelude
+
+concrete Toy0Fin of Toy0 = Toy0I with
+  (Syntax = SyntaxFin),
+  (Lexicon = LexiconFin) ;