lapply or data.table to find a unit's previous transaction
An embedded and charset-unspecified text was scrubbed... Name: not available URL: <https://stat.ethz.ch/pipermail/r-sig-hpc/attachments/20100602/b6aeb738/attachment.pl>
An embedded and charset-unspecified text was scrubbed... Name: not available URL: <https://stat.ethz.ch/pipermail/r-sig-hpc/attachments/20100602/b6aeb738/attachment.pl>