visit method
Implementation
void visit(Envelope? searchEnv, ItemVisitor visitor) {
if(searchEnv == null){
int stop = 1;
}
if (!isSearchMatch(searchEnv)) return;
// this node may have items as well as subnodes (since items may not
// be wholely contained in any single subnode
visitItems(searchEnv!, visitor);
for (int i = 0; i < 4; i++) {
if (subnode[i] != null) {
subnode[i]!.visit(searchEnv, visitor);
}
}
}