Add numbers.

language.grammar

@@ -1,6 +1,7 @@
 token whitespace = /[ \n\t]+/ [ skip ];
 token upper_identifier = /[A-Z_][a-z_A-Z0-9]*/;
 token lower_identifier = /[a-z_][a-z_A-Z0-9]*/;
+token number = /[0-9]+/;
 token kw_data = /data/;
 token kw_type = /type/;
 token kw_defn = /defn/;
@@ -90,6 +91,7 @@ rule expr_app
 rule expr_app_bottom
     = lower_identifier
     | upper_identifier
+    | number
     | tkn_oparen expr tkn_cparen
     ;