I was reading this article.
And got stuck understanding this derivation:
Consider the alphabet $\Sigma = \{a,b,c\}$. A possible dependency relation is \begin{array}{ccc} D&=&\{a,b\}\times\{a,b\}\quad\cup\quad\{a,c\}\times\{a,c\}\\ &=&\{a,b\}^2\cup\{a,c\}^2\\ &=&\{(a,b),(b,a),(a,c),(c,a),(a,a),(b,b),(c,c)\}. \end{array}
How can a union of square of 2 sets derived to the 3rd line?
PS: I'm not from Maths background.