problem defining a set

[jack malkovick‏]

Hello. I'm trying to define a set like this

{ a + b : a \in 0..2, b \in 0..a }

I know it's wrong, because a in b \in 0..a is unknown, but how could I do it in another way?

[Leslie Lamport‏]

UNION {{ a + b : b \in 0..a}: a \in 0..2}

[jack malkovick‏]

Elegant, thank you!