tweak(parikh_image): slightly tighten fixed optimisation

src/main/parikh_image_one.cpp

@@ -200,7 +200,8 @@ expr const ParikhImageOne::build_var_domains_formula() {
 
                     // either different to prev or ccount is 0
                     domains = domains && (
-                        (prev_char < curr_char) || ccount == 0
+                        (prev_char < curr_char)
+                        || (ccount == 0 && prev_char == curr_char)
                     );
                     expr some_diff = context.bool_val(false);
                     for (expr const &pred : predicates) {