Function: listput
Section: programming/specific
C-Name: listput0
Prototype: WGD0,L,
Help: listput(list,x,{n}): sets n-th element of list equal to x. If n is
 omitted or greater than the current list length, appends x.
Description:
 (list, gen, small):gen        listput($1, $2, $3)
Doc:
 sets the $n$-th element of the list
 \var{list} (which must be of type \typ{LIST}) equal to $x$. If $n$ is omitted,
 or greater than the list length, appends $x$.
 You may put an element into an occupied cell (not changing the
 list length), but it is easier to use the standard \kbd{list[n] = x}
 construct. This runs in time $O(\#L)$ in the worst case (when the list must
 be reallocated), but in time $O(1)$ on average: any number of successive
 \kbd{listput}s run in time $O(\#L)$, where $\#L$ denotes the list
 \emph{final} length.