XSetBackgroundPixmap question

[James Buren]

When using this Xlib function to set a new background pixmap, how does the old one get
freed? If I am setting the root window's background pixmap, I don't know how I can get the
XID of the old one. If it is a window I created, I can cache it but that seems overkill if I'm
just wanting to discard the old one. Can someone direct me to how I am supposed to free
the old background pixmap? Thanks.