When an application has finished using a screen, the memory that the
screen occupied should be returned to the system by calling CloseScreen().
Normally, an application should close only those screens that it created.
Under V34 and earlier versions of the OS, CloseScreen() returns no values.
Under Release 2, CloseScreen() returns a boolean value, TRUE for success
and FALSE for failure.  CloseScreen() can fail if the screen is public and
another task is still using the screen.