Default value: nil

X11/Motif applications only: if the application uses the X11 code or CAPI, it may fail to run if it cannot open the X display.

In this case, if the value is a function it calls this function with one argument, the display name. The default value of nil means that a message is printed and Lisp quits.

LispWorks Delivery User Guide - 12 Sep 2005