[S32::Exception] Add X::Syntax::Term::MissingInitializer

S32-setting-library/Exception.pod

@@ -765,6 +765,14 @@ example C<rx:g/a/>).
         method message() { "Adverb $.adverb not allowed on $.construct" }
     }
 
+=head3 X::Syntax::Term::MissingInitializer
+
+Thrown when a backslash-variable is declared without initialization assignment.
+
+    my class X::Syntax::Term::MissingInitializer does X::Syntax {
+        method message { "Term definition requires an initializer" }
+    }
+
 =head3 X::Syntax::Signature::InvocantMarker
 
 Thrown when the invocant in a signature is anywhere else than after the