Sets iter to be the child of parent, using the given index. The first
index is 0. If n is too big, or parent has no children, iter is set
to an invalid iterator and FALSE is returned. parent will remain a valid
node after this function has been called. As a special case, if parent is
NULL, then the nth root node is set.
Sets iter to be the child of parent, using the given index. The first index is 0. If n is too big, or parent has no children, iter is set to an invalid iterator and FALSE is returned. parent will remain a valid node after this function has been called. As a special case, if parent is NULL, then the nth root node is set.