diff --git a/include/qasm/AST/ASTBase.h b/include/qasm/AST/ASTBase.h
index e11dc49..4f4689e 100644
--- a/include/qasm/AST/ASTBase.h
+++ b/include/qasm/AST/ASTBase.h
@@ -201,12 +201,10 @@ class ASTTokenFactory {
   }
 
   static const ASTToken *GetPreviousToken() {
-    std::map<uint32_t, ASTToken *>::const_reverse_iterator MRI = TFM.rbegin();
-    if (MRI == TFM.rend())
+    if (TFM.empty())
       return nullptr;
 
-    --MRI;
-    return MRI == TFM.rend() ? nullptr : (*MRI).second;
+    return std::prev(TFM.end())->second;
   }
 
   static uint32_t GetCurrentIndex() { return TIX; }