Concatenation

The concatenation of words $x$ and $y$ is the word $x$ followed by the word $y$, written $xy$. We can use the syntax $\text{foo}@\text{bar}=\text{foobar}$.

Formally, $A@B\stackrel{def}{=}\{s_1@s_2|s_1\in A\wedge s_2\in B\}$.