Write two functions that demonstrate logging and rethrowing an exception:

1. loadFile(const string& filename): if filename is empty, throw a std::runtime_error with "Empty filename". Otherwise print "Loading: " followed by filename.

2. openDocument(const string& filename): calls loadFile inside a try block. If a runtime_error is caught, print "[LOG] " followed by e.what(), then rethrow the same exception using throw; (bare throw with no argument).

The caller of openDocument handles the final exception.

Do NOT write a main function.