Add column that restarts counting for each new category

Hi all,

I am trying to add a number column that restarts counting every time a new category starts.

Example:

Cat  CustomColumn

A     1

A     2

A     3

B     1

B     2

B     3

C     1

C     2

C     3

In Excel, this is very easy, but in Power Query I have a hard time achieving this.

Any ideas?

Thanks,

Kirvis