How to Pass MULTIPLE filenames to a Context Menu Shell Command?

Passing a single filename to a context menu shell command is simple:

[HKEY_CLASSES_ROOT\*\shell\MyProgram\Command]
@="program.exe %1"

But if I select multiple files, program.exe is invoked for each such selected file.

What I would like to do instead is invokeprogram.exe only once, passing to it all the filenames currently selected.

How to do this?