Looks like it ought to still be a thing:

http://h30266.www3.hpe.com/odl/vax/opsys/vmsos73/vmsos73/648...

> "You can abbreviate a command as long as the abbreviated name remains unique among the defined commands on a system. DCL looks only at the first four characters for uniqueness."

and since max command line length seems to be a few K, the masochistic could try instead:

    $ DISCIPLINEOFPROGRAMMING

(are there any other 1976 things in the equivalence set of DISCONNECT?)