How to add "sdfopen" and "xdfopen" commands into my GrADS

[Ming Yang]

Hi, folks
    My GrADS version is 1.8SL11. I would like to use it to read some
NetCDF files. I learn from the manual that I can read these files by using
sdfopen or xdfopen. But the problem is when I type sdfopen or xdfopen in
my GrADS session, it is said that they are Unknown commands. Does anybody
know why this error happen? Do these two commands come with the package or
I have to add them by myself? And How? Thank you for your attention.

Best,
Ming