MDX help - case expression based on aggregated value

I need to solve the following problem (I'm a newbie on MDX) :

FactTable
Year:int
Type: "A" | "B"
M1:int

How to compute (MDX) the following expression:

based on the current aggregation, IF the sum of M1 > 3 THEN sum of M1
ELSE "NOT PUBLISHABLE"

So for example:
FactTable
Year Type M1 ComputedVal
--------------------------------------------
2000 A 1 "NOT PUBLISHABLE"
2000 B 2 "NOT PUBLISHABLE"

2001 A 1 "NOT PUBLISHABLE"
2001 B 5 5

2002 A 1 "NOT PUBLISHABLE"
2002 B 13 13


If aggregation is by Year, then we want to obtain:

Year Type M1 ComputedVal
--------------------------------------
2000 - 3 "NOT PUBLISHABLE" <--- how to ?
2001 - 6 6
2002 - 14 14


If aggregation is by Type, then we want to obtain:

Year Type M1 ComputeVal
---------------------------------------
- A 3 "NOT PUBLISHABLE" <-- how to ?
- B 20 20