ok
off topic
who knows what that means
"Concatenation is a combination of sets to form a new set. The linked set consists of all combinations of the elements of both sets using a normally non-commutative operation. The operation used is usually the concatenation of the elements.
Concatenation is a modification of the product set operation (Cartesian product), neglecting the Tuple notation. In formal terms, concatenation and the Cartesian product are therefore different. However, as this is only a notation, concatenation is identified with the Cartesian product in SQL, for example."

I love Shiraz.)