How to remove item from picklist which have been selected in previous entries in a subform context?

How to remove item from picklist which have been selected in previous entries in a subform context?

Hi! I have a subform with two fields: Item and Qty. Item is a dynamic picklist.  I need to remove item from the picklist every time I add a new row. I'm stuck as I cannot get the picklist to work.

Here is my code:

\\This part is where I count the number of entries in my subform\\
On Add Row
for each recA in Collection_Tracking
{
    recCount = 0;
    for each recB in Collection_Items
    {
        recCount = (recCount  +  1);
    }
}
\\
\\
\\UsedList is a List variable updated with a Multi-Line field called UsedItem.
UsedList = List();
UsedList = input.UsedItem.toList();
alert "UsedList is " + UsedList; \\this is for testing purposes\\

if ((input.Customer_Name  !=  "")  &&  (input.Customer_Name  !=  "-Select-"))
{
    clear row.Item;
    if (recCount  =  1)
    {
        \\for the first record, the picklist takes the full list
        List  =  Items  [Customer_Name == input.Customer_Name];
        row.Item:ui.append(List.Item_Name.getall());
    }
    else
    {
      \\for the rest of the records, the items selected before (saved in UsedList) are meant to be removed from the list
        List  =  Items  [(Customer_Name == input.Customer_Name && (Item_Name not in UsedList))];
        row.Item:ui.append(List.Item_Name.getall());
       
    }
    disable Customer_Name;
    row.Collection_ID = input.Collection_ID;
}
else
{
    reload;
    alert("Enter Customer Name First!");
}


Here is my code for the field "Item" (my picklist)

on user input
                {
                    WorkingList = List();
                    WorkingList = input.UsedItem.toList();
                    temp = "\"" + row.Item + "\"";
                    WorkingList.add(temp.toString());
                    alert("workingList after adding the current item is " + WorkingList);
                    input.UsedItem = WorkingList.toString();
                    alert(input.UsedItem);
                }


Please help. I'm stuck for hours and hours on this.