; called as part of a screen ACTION PROTOCOL'S ENTRY ACTION to determine if screen has already been displayed
; returns true if screen array already exists (ie. already displayed),
; setting IBFASTXT causes LM to back out of current screens,
; setting IBPRVSCR causes LM to stop exiting screens when the chosen screen is reached
; if user tries to execute a screen already displayed it will quit out of existing screens until the asked for screen is found