Thanks. Reason I asked about removing an item from a purchase list is to 'clean up' the UI; and not confuse the player by having purchase options for lots of items with similar characteristics, e.g.
AIM-170A Gryphon
AIM-170B Gryphon
AIM-170C Gryphon
which unlock as purchases as the player researches more of the alien UFO subsystems and can more effectively design conventional earth weaponry as you now know more about what goes on around an UFO and can design weaponry to see UFOs better or survive the area around UFOs better.