How to iterate over a column that has different categorical groups?

[#1]

Let's say column called Lot has 3 different entries. And that I wanted to add a logic to create a column called "Box Start" that checks which box runs on sequence 1 and copies that information for all entries of that lot in a column called "Box Start" like how it is done below. 

How can I design a for loop to perform this operation?

Lot	Sequence	Box	Box Start
A	1	a	Starts on a
A	2	b	Starts on a
A	3	a	Starts on a
A	4	b	Starts on a
B	1	b	Starts on b
B	2	b	Starts on b
B	3	a	Starts on b
B	4	a	Starts on b
B	5	a	Starts on b
C	1	a	Starts on a
C	2	b	Starts on a
C	3	a	Starts on a

[#2]

Here is one way to solve the problem;  I created a formula that whenever a new value of Lot is found, it calculates when the first occurrence of Lot appeared.

If( :Lot != Lag( :Lot ),
	currLot = :Lot;
	begin = :Box[Min( Current Data Table() << get rows where( :Lot == currLot ) )];
	
);
begin;

If you want to iterate in open code, the following code can be used

For Each Row(
	If( :Lot != Lag( :Lot ),
		currLot = :Lot;
		begin = :Box[Min( Current Data Table() << get rows where( :Lot == currLot ) )];
	);
	Show( begin );
);