Names a new function to handle events, called in response to handle-key-event, handle-key-event, or call-function. The return value is of the procedure is ignored.
If there was already a function mapped to this name, it will be replaced with the given function.
When the function is called, it gets the arguments that were passed to handle-key-event, handle-mouse-event, or call-function. For keymaps asscociated with an editor, this is normally the target editor.