Abort 

Hi,

I ran accuMUlate and got an error:

```
terminate called after throwing an instance of 'std::out_of_range'
  what():  basic_string::substr: __pos (which is 18446744073709551615) > this->size() (which is 60)
Aborted
```
It seems somewhere wrong in Cpp codes. Any ideas how to fix it?

Thanks very much.