HOW to reset the LineNum?

hi,

In BOM form one from is there.i.e,LineNUM…if user deletes any one line,all the number should reste

ex:-In Bill of Material there are 5 lines with serial number 01 to 05. In case user deletes the row with serial number 02, in that case system should reset the Serial Number from 01 to 04.

regards,

sandesh