Can I use the Lag function against a custom member - mdx

Question

In the following I successfully find the last complete month and the billable income related to that month. Then I'd like to target the month before and find the income for that month. Thought that Lag would be a good function to use but this doesn't seem to work. No errors just no data returned for the second column [Measures].[PrevMth_BillInc]:
WITH
MEMBER [Date].[Date - Calendar Month].[LastMth] AS
IIF(
Day(Now()) = 1,
TAIL([Date].[Date - Calendar Month].[Calendar Month],2).Item(1),
TAIL([Date].[Date - Calendar Month].[Calendar Month],2).Item(0)
)
MEMBER [Measures].[LastMth_BillInc] AS
SUM(
[Date].[Date - Calendar Month].[LastMth],
[Measures].[BillableIncome]
)
MEMBER [Measures].[PrevMth_BillInc] AS
SUM(
[Date].[LastMth].Lag(1),
[Measures].[BillableIncome]
)
SELECT
NON EMPTY
{
[Measures].[LastMth_BillInc],
[Measures].[PrevMth_BillInc] //<<<<<this returns nothing
}
ON COLUMNS,
NON EMPTY
ORDER(
[Customer_Dim].[Customer_Hier].[Customer_Level].Members,
[Date].[Date - Calendar Month].[LastMth],
BDESC
)
ON ROWS
FROM [ourCube]

Answer 1

By defining [LastMth] as a calculated member, you define a new member of the [Date].[Date - Calendar Month], not an alias to an existing one. Hence Lag does not work, as it is applied to the new member - which is somewhere in the hierarchy alongside the original member. And as Lag is a function which operates on the hierarchy structure, it is not to the original member used in its definition.
You could get around that by defining LastMth as a set which would contain the original, non calculated member (to be exact, it would contain a tuple containing the original member) as follows:
WITH
SET [LastTwo] AS
TAIL([Date].[Date - Calendar Month].[Calendar Month],2)
SET [LastMonth] AS
IIF(
Day(Now()) = 1,
Subset([LastTwo], 1, 1),
Subset([LastTwo], 0, 1)
)
MEMBER [Measures].[LastMth_BillInc] AS
(
[LastMth].Item(0).Item(0),
[Measures].[BillableIncome]
)
MEMBER [Measures].[PrevMth_BillInc] AS
(
[LastMth].ITEM(0).Item(0).Lag(2),
[Measures].[BillableIncome]
)
SELECT
NON EMPTY
{
[Measures].[LastMth_BillInc],
[Measures].[PrevMth_BillInc]
}
ON COLUMNS,
NON EMPTY
ORDER(
[Customer_Dim].[Customer_Hier].[Customer_Level].Members,
[Date].[Date - Calendar Month].[LastMth],
BDESC
)
ON ROWS
FROM [ourCube]
I think you do not need sum, you can just use tuples as I did above, as there is only one member and not a set of several to be summed.