Fix empty formula generation for self-recursive functions (#631)

Author: Copilot

src/form/formula_simplify.cpp

@@ -389,6 +389,13 @@ bool isSimpleRecursiveReference(const Formula& formula,
   }
 
   refFuncName = rhs.name;
+
+  // Skip if the function references itself (self-recursion, not a simple
+  // reference)
+  if (refFuncName == funcName) {
+    return false;
+  }
+
   const auto& arg = rhs.children.front();
 
   // Skip if the referenced function was already processed