Address comments from review

src/visitors/cvode_visitor.cpp

@@ -153,10 +153,10 @@ class StiffVisitor: public CvodeHelperVisitor {
 
         auto rhs = node.get_rhs();
         // all indexed variables (need special treatment in SymPy)
-        auto name_map = get_indexed_variables(*rhs, name->get_node_name());
+        auto indexed_variables = get_indexed_variables(*rhs, name->get_node_name());
         auto diff2c = pywrap::EmbeddedPythonLoader::get_instance().api().diff2c;
-        auto [jacobian,
-              exception_message] = diff2c(to_nmodl(*rhs), parse_independent_var(name), name_map);
+        auto [jacobian, exception_message] =
+            diff2c(to_nmodl(*rhs), parse_independent_var(name), indexed_variables);
         if (!exception_message.empty()) {
             logger->warn("CvodeVisitor :: python exception: {}", exception_message);
         }
@@ -172,11 +172,10 @@ class StiffVisitor: public CvodeHelperVisitor {
     }
 };
 
-
-void CvodeVisitor::visit_program(ast::Program& node) {
+static std::shared_ptr<ast::DerivativeBlock> get_derivative_block(ast::Program& node) {
     auto derivative_blocks = collect_nodes(node, {ast::AstNodeType::DERIVATIVE_BLOCK});
     if (derivative_blocks.empty()) {
-        return;
+        return nullptr;
     }
 
     // steady state adds a DERIVATIVE block with a `_steadystate` suffix
@@ -195,8 +194,15 @@ void CvodeVisitor::visit_program(ast::Program& node) {
         throw std::runtime_error(message);
     }
 
-    auto derivative_block = std::dynamic_pointer_cast<ast::DerivativeBlock>(
-        derivative_blocks_copy[0]);
+    return std::dynamic_pointer_cast<ast::DerivativeBlock>(derivative_blocks_copy[0]);
+}
+
+
+void CvodeVisitor::visit_program(ast::Program& node) {
+    auto derivative_block = get_derivative_block(node);
+    if (derivative_block == nullptr) {
+        return;
+    }
 
     auto non_stiff_block = derivative_block->get_statement_block()->clone();
     remove_conserve_statements(*non_stiff_block);