add clarification *mutable* in std iterators vs *mutable* in podio

doc/collections_as_container.md

@@ -121,11 +121,14 @@ The C++ specifies a set of named requirements for iterators. Starting with C++20
 
 ### LegacyForwardIterator
 
+In addition to the *LegacyForwardIterator* the C++ standard specifies also the *mutable LegacyForwardIterator*, which is both *LegacyForwardIterator* and *LegacyOutputIterator*. The term **mutable** used in this context doesn't imply mutability in the sense used in the PODIO.
+
+
 | Requirement | Fulfilled by `iterator`/`const_iterator`? | Comment |
 |-------------|-------------------------------------------|---------|
 | [*LegacyInputIterator*](https://en.cppreference.com/w/cpp/named_req/InputIterator) | ❌ no / ❌ no | [See above](#legacyinputiterator)|
 | [*DefaultConstructible*](https://en.cppreference.com/w/cpp/named_req/DefaultConstructible) | ❌ no / ❌ no | Value initialization not defined |
-| If mutable iterator then `reference` same as `value_type&` or `value_type&&`, otherwise same as `const value_type&` or `const value_type&&` | ❌ no / ❌ no | `reference` and `value_type` not defined |
+| If *mutable* iterator then `reference` same as `value_type&` or `value_type&&`, otherwise same as `const value_type&` or `const value_type&&` | ❌ no / ❌ no | `reference` and `value_type` not defined |
 | [Multipass guarantee](https://en.cppreference.com/w/cpp/named_req/ForwardIterator) | ❌ no / ❌ no | Copy constructor not defined |
 | [Singular iterators](https://en.cppreference.com/w/cpp/named_req/ForwardIterator) | ❌ no / ❌ no | Value initialization not defined |
 

tests/unittests/std_interoperability.cpp

@@ -590,14 +590,33 @@ TEST_CASE("Collection iterators", "[collection][container][iterator][std]") {
 
     } // end of LegacyInputIterator
 
-    // Mutable iterator: reference same as value_type& or value_type&&
+    // Mutable LegacyForwardIterator (LegacyForwardIterator that is also LegacyOutputIterator):
+    // - reference same as value_type& or value_type&&
+    // iterator
     DOCUMENTED_STATIC_FAILURE(traits::has_reference_v<iterator>);
     DOCUMENTED_STATIC_FAILURE(traits::has_value_type_v<iterator>);
     // STATIC_REQUIRE(
     //     std::is_same_v<std::iterator_traits<iterator>::reference, std::iterator_traits<iterator>::value_type&> ||
     //     std::is_same_v<std::iterator_traits<iterator>::reference, std::iterator_traits<iterator>::value_type&&>);
+    // const_iterator
+    DOCUMENTED_STATIC_FAILURE(traits::has_reference_v<const_iterator>);
+    DOCUMENTED_STATIC_FAILURE(traits::has_value_type_v<const_iterator>);
+    // STATIC_REQUIRE(
+    //     std::is_same_v<std::iterator_traits<const_iterator>::reference,
+    //     std::iterator_traits<const_iterator>::value_type&> ||
+    //     std::is_same_v<std::iterator_traits<const_iterator>::reference,
+    //     std::iterator_traits<const_iterator>::value_type&&>);
 
-    // Immutable iterator: reference same as const value_type& or const value_type&&
+    // (Immutable) iterator:
+    // - reference same as const value_type& or const value_type&&
+    // iterator
+    DOCUMENTED_STATIC_FAILURE(traits::has_reference_v<iterator>);
+    DOCUMENTED_STATIC_FAILURE(traits::has_value_type_v<iterator>);
+    // STATIC_REQUIRE(std::is_same_v<std::iterator_traits<iterator>::reference,
+    //                               const std::iterator_traits<iterator>::value_type&> ||
+    //                std::is_same_v<std::iterator_traits<iterator>::reference,
+    //                               const std::iterator_traits<iterator>::value_type&&>);
+    // const_iterator
     DOCUMENTED_STATIC_FAILURE(traits::has_reference_v<const_iterator>);
     DOCUMENTED_STATIC_FAILURE(traits::has_value_type_v<const_iterator>);
     // STATIC_REQUIRE(std::is_same_v<std::iterator_traits<const_iterator>::reference,