void fetchPrev(SpanItr &s){
if(s-- == spans.begin()) {
uint64_t prvInterval=prevInterval(s->first, timeUnit);
s=spans.insert(std::pair<int64_t,spanView*>(prvInterval, new spanView(this, prvInterval, s->first))).first;
}
};
SpanItr fetchSpan(int64_t idx){
SpanItr ret=spans.find(idx);
if(ret!=spans.end()) return ret;
spans.insert(std::pair<int64_t,spanView*>(idx, new spanView(this, idx, nextInterval(idx,timeUnit)))).first;
};
void fetchNext(SpanItr &s){
if(++s==spans.end()) {
uint64_t nxtInterval=nextInterval(s->first, timeUnit);
s=spans.insert(std::pair<int64_t,spanView*>(nxtInterval, new spanView(this, s->first,nxtInterval))).first;
}
};
// The 'move' commands fetch spans as needed then set leftMostSpan and backLeft.
void moveTo(uint64_t idx){
SpanItr ret=spans.find(idx);
if(ret!=spans.end()) leftMostSpan=ret;
else leftMostSpan=spans.insert(std::pair<int64_t,spanView*>(idx, new spanView(this, idx, nextInterval(idx,timeUnit)))).first;
backLeft=0;
};