title: generate serial No with arrayformula relative to previous column
body: <p>i'm trying to get dynamic serial number for my accounts tree excluding blanks and relative to the previous column but i couldn't mange to achieve that with arrayformula
  <a href="https://docs.google.com/spreadsheets/d/1Y_Zfwos5KDbY8EbHY2euUd6HjYOmpOm0g3QF_e5Eqj4/edit?usp=sharing" rel="nofollow noreferrer">copy of my sheet</a></p>