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;
};
// 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;
};