I am trying to find out how to insert umlauts in TeXmacs using the emacs look-and-feel, and I'm having problems.

The documentation page on general prefix rules contains the following paragraph:

"Unfortunately, CMD-based shortcuts are superseded by system shortcuts on several systems. For instance, accented characters and common special symbols are entered using this prefix under Mac OS. In that case, you may use the ??? key as an equivalent for CMD. For more information, we refer to the section on keyboard configuration."

In place of ??? above, the key that is supposed to act as a replacement for CMD, there is an icon of an arrow pointing diagonally left up from the middle of a circle. Which key is this?

Thank you very much,