cvs-prefix-set is a compiled Lisp function in `pcvs-util.el'.

(cvs-prefix-set SYM ARG)

Set the cvs-prefix contained in SYM.
If ARG is between 0 and 9, it selects the corresponding default.
If ARG is negative (or C-u which corresponds to negative 0),
it queries the user and sets the -ARGth default.
If ARG is greater than 9 (or C-u C-u),
the (ARG mod 10)'th prefix is made persistent.
If ARG is nil toggle the PREFIX's value between its 0th default and nil
and reset the persistence.