ToobPlayer UI. .mdata file handling.

PiPedalCommon/src/include/json_variant.hpp

@@ -309,6 +309,10 @@ namespace pipedal
         const_iterator begin() const { return values.begin(); }
         const_iterator end() const { return values.end(); }
 
+        iterator erase(iterator it)
+        {
+            return values.erase(it);
+        }
         iterator find(const std::string &key);
         const_iterator find(const std::string &key) const;