Overwrite builtin Functions



I am trying to get help this way. I hope the answer appears in my mailbox.
I am not subscribed to the list.

How can I overwrite a Maxima functions, e.g. to extend its functionality,
something like

hessian(f,[v]) := if length(v)=0 then hessian(f,listofvars(f)) else
hessian(f,v[1])

Wouldn't that be a recursive definition?

Rene Grothmann