This function deletes a previously defined deffunction.
Syntax
(undeffunction <deffunction-name>)
If the symbol * is used for <deffunction-name>, then all
deffunctions will be deleted (unless there exists a deffunction
called *). The undeffunction command can be used to remove deffunctions
at any time. Exceptions: A deffunction may not be deleted when
it is executing or when there is still a reference to it in another
loaded construct, such as a rule RHS. This function has no return
value.