Function: listpop
Section: programming/specific
C-Name: listpop0
Prototype: vWD0,L,
Help: listpop(list,{n}): removes n-th element from list. If n is
 omitted or greater than the current list length, removes last element.
Description:
 (list, small):void     listpop($1, $2)
Doc:
 removes the $n$-th element of the list
 \var{list} (which must be of type \typ{LIST}). If $n$ is omitted,
 or greater than the list current length, removes the last element.
 If the list is already empty, do nothing. This runs in time $O(\#L - n + 1)$.
