Generate unique number while reading in segments

Summary

How to generate a unique number in the xslt while reading the file using segment read from stage

Content

Hi, 

I am reading an invoice file using the AIC stage read activity. Since the file size is more than 10MB we need to read the file in segments. While transforming the data, it is required to generate a unique invoice id for each line. Tried to concatenate the instance id with the position. But since each segment will have 200 records each, the position is getting repeated in the segments and hence getting unique constraint issue. 

Any suggestion is appreciated.