What does Delta mean in this case

[rad0786]

Homework Statement

R = { (X,Y) $$\epsilon$$ P(A) x P(A) / (X$$\Delta$$Y) $$\subseteq$$ B}

(X$$\Delta$$Y) $$\subseteq$$ B - what does delta mean?

Homework Equations

The Attempt at a Solution

 

[HallsofIvy]

Probably "symmetric difference": A union B \ A intersect B: that is, all points that are in A or B but [/b]not[/b] in both.